diff --git a/server/actions/client/client.go b/server/actions/client/client.go
new file mode 100644
index 0000000..b391d6e
--- /dev/null
+++ b/server/actions/client/client.go
@@ -0,0 +1,19 @@
+package client
+
+import "hexmap-server/actions/syncable"
+
+type Hello struct {
+	Version int `json:"version"`
+}
+
+type Refresh struct {
+}
+
+type SentAction struct {
+	Id     int             `json:"id"`
+	Action syncable.Action `json:"action"`
+}
+
+type SentActions struct {
+	Nested []SentAction `json:"nested"`
+}
diff --git a/server/actions/server/server.go b/server/actions/server/server.go
new file mode 100644
index 0000000..abb4e43
--- /dev/null
+++ b/server/actions/server/server.go
@@ -0,0 +1 @@
+package server
diff --git a/server/actions/syncable/action.go b/server/actions/syncable/action.go
new file mode 100644
index 0000000..4b2f701
--- /dev/null
+++ b/server/actions/syncable/action.go
@@ -0,0 +1,18 @@
+package syncable
+
+import (
+	"errors"
+	"hexmap-server/state"
+)
+
+var ErrorNoOp error = errors.New("action's effects were already applied, or it's an empty action")
+
+// Action is the interface for actions that can be shared.
+type Action interface {
+	// Apply causes the action's effects to be applied to s, mutating it in place.
+	// All syncable.Actions must conform to the standard that if an action can't be correctly applied, or if it would
+	// have no effect, it returns an error without changing s.
+	// If an action can be correctly applied but would have no effect, it should return ErrorNoOp.
+	// If an action is correctly applied and has an effect, it should return nil.
+	Apply(s *state.Synced) error
+}
diff --git a/server/go.mod b/server/go.mod
new file mode 100644
index 0000000..bdf7d9b
--- /dev/null
+++ b/server/go.mod
@@ -0,0 +1,5 @@
+module hexmap-server
+
+go 1.16
+
+require github.com/rs/xid v1.3.0 // indirect
diff --git a/server/go.sum b/server/go.sum
new file mode 100644
index 0000000..47ea790
--- /dev/null
+++ b/server/go.sum
@@ -0,0 +1,2 @@
+github.com/rs/xid v1.3.0 h1:6NjYksEUlhurdVehpc7S7dk6DAmcKv8V9gG0FsVN2U4=
+github.com/rs/xid v1.3.0/go.mod h1:trrq9SKmegXys3aeAKXMUTdJsYXVwGY3RLcfgqegfbg=
diff --git a/server/persistence/persistence.go b/server/persistence/persistence.go
new file mode 100644
index 0000000..dc7cf83
--- /dev/null
+++ b/server/persistence/persistence.go
@@ -0,0 +1 @@
+package persistence
diff --git a/server/state/coordinates.go b/server/state/coordinates.go
new file mode 100644
index 0000000..4608586
--- /dev/null
+++ b/server/state/coordinates.go
@@ -0,0 +1,9 @@
+package state
+
+// StorageCoordinates gives the coordinates of a cell in a form optimized for storage.
+type StorageCoordinates struct {
+	// Line is the index from 0 to Lines - 1 of the HexLine in the HexLayer.
+	Line uint8 `json:"line"`
+	// Cell is the index from 0 to CellsPerLine - 1 of the HexCell in the HexLine.
+	Cell uint8 `json:"cell"`
+}
diff --git a/server/state/hexcolor.go b/server/state/hexcolor.go
new file mode 100644
index 0000000..9f2a5c9
--- /dev/null
+++ b/server/state/hexcolor.go
@@ -0,0 +1,104 @@
+package state
+
+import "fmt"
+
+// HexColor is an internal representation of a hexadecimal color string.
+// It can take one of these formats:
+// #RRGGBBAA
+// #RRGGBB
+// #RGBA
+// #RGB
+// When marshaling, it will always choose the most efficient format, but any format can be used when unmarshaling.
+type HexColor struct {
+	// R is the red component of the color.
+	R uint8
+	// G is the green component of the color.
+	G uint8
+	// B is the blue component of the color.
+	B uint8
+	// A is the alpha component of the color.
+	A uint8
+}
+
+// HexColorFromString decodes a hexadecimal string into a hex color.
+func HexColorFromString(text string) (HexColor, error) {
+	var hex HexColor
+	if err := (&hex).UnmarshalText([]byte(text)); err != nil {
+		return hex, err
+	}
+	return hex, nil
+}
+
+func (h *HexColor) UnmarshalText(text []byte) error {
+	var count, expected int
+	var short bool
+	var scanErr error
+	switch len(text) {
+	case 9:
+		// Long form with alpha: #RRGGBBAA
+		expected = 4
+		short = false
+		count, scanErr = fmt.Sscanf(string(text), "#%02X%02X%02X%02X", &h.R, &h.G, &h.B, &h.A)
+	case 7:
+		// Long form: #RRGGBB
+		expected = 3
+		h.A = 0xFF
+		count, scanErr = fmt.Sscanf(string(text), "#%02X%02X%02X", &h.R, &h.G, &h.B)
+	case 5:
+		// Short form with alpha: #RGBA
+		expected = 4
+		short = true
+		count, scanErr = fmt.Sscanf(string(text), "#%01X%01X%01X%01X", &h.R, &h.G, &h.B, &h.A)
+	case 4:
+		// Short form: #RGB
+		expected = 3
+		h.A = 0xF
+		short = true
+		count, scanErr = fmt.Sscanf(string(text), "#%01X%01X%01X", &h.R, &h.G, &h.B)
+	default:
+		return fmt.Errorf("can't decode %s as HexColor: wrong length", text)
+	}
+	if scanErr != nil {
+		return scanErr
+	}
+	if count != expected {
+		return fmt.Errorf("can't decode %s as HexColor: missing components", text)
+	}
+	if short {
+		h.R *= 0x11
+		h.G *= 0x11
+		h.B *= 0x11
+		h.A *= 0x11
+	}
+	return nil
+}
+
+// MarshalText marshals the HexColor into a small string.
+func (h HexColor) MarshalText() (text []byte, err error) {
+	return []byte(h.String()), nil
+}
+
+// String prints the HexColor as an abbreviated notation.
+// Specifically, short form is used when possible (i.e., when all components are evenly divisible by 0x11).
+// The alpha component is left out if it's 0xFF.
+func (h HexColor) String() string {
+	if h.R%0x11 == 0 && h.G%0x11 == 0 && h.B%0x11 == 0 && h.A%0x11 == 0 {
+		// Short form works.
+		if h.A == 0xFF {
+			// It's great when it's easy!
+			return fmt.Sprintf("#%01X%01X%01X", h.R/0x11, h.G/0x11, h.B/0x11)
+		} else {
+			// Just need to add the alpha.
+			return fmt.Sprintf("#%01X%01X%01X%01X", h.R/0x11, h.G/0x11, h.B/0x11, h.A/0x11)
+		}
+	} else {
+		// Gotta use long form.
+		if h.A == 0xFF {
+			// Can skip the alpha channel, though.
+			return fmt.Sprintf("#%02X%02X%02X", h.R, h.G, h.B)
+		} else {
+			// Doing things the hard way.
+			return fmt.Sprintf("#%02X%02X%02X%02X", h.R, h.G, h.B, h.A)
+		}
+	}
+}
diff --git a/server/state/hexmap.go b/server/state/hexmap.go
new file mode 100644
index 0000000..b9282cb
--- /dev/null
+++ b/server/state/hexmap.go
@@ -0,0 +1,83 @@
+package state
+
+import (
+	"fmt"
+	"github.com/rs/xid"
+)
+
+// HexMapRepresentation combines HexOrientation and LineParity to represent a map's display mode.
+type HexMapRepresentation struct {
+	Orientation   HexOrientation `json:"orientation"`
+	IndentedLines LineParity     `json:"indentedLines"`
+}
+
+// HexCell contains data for a single cell of the map.
+type HexCell struct {
+	// Color contains the color of the cell, in hex notation.
+	Color HexColor `json:"color"`
+}
+
+// HexLine is a line of cells which are adjacent by flat sides in a vertical or horizontal direction.
+type HexLine []HexCell
+
+// Copy creates a deep copy of this HexLine.
+func (l HexLine) Copy() HexLine {
+	duplicate := make(HexLine, len(l))
+	for index, value := range l {
+		duplicate[index] = value
+	}
+	return duplicate
+}
+
+// HexLayer is a two-dimensional plane of cells which are arranged into lines.
+type HexLayer []HexLine
+
+// GetCellAt returns a reference to the cell at the given coordinates.
+func (l HexLayer) GetCellAt(c StorageCoordinates) (*HexCell, error) {
+	if int(c.Line) > len(l) {
+		return nil, fmt.Errorf("line %d out of bounds (%d)", c.Line, len(l))
+	}
+	line := l[c.Line]
+	if int(c.Cell) > len(line) {
+		return nil, fmt.Errorf("cell %d out of bounds (%d)", c.Cell, len(line))
+	}
+	return &(line[c.Cell]), nil
+}
+
+// Copy creates a deep copy of this HexLayer.
+func (l HexLayer) Copy() HexLayer {
+	duplicate := make(HexLayer, len(l))
+	for index, value := range l {
+		duplicate[index] = value.Copy()
+	}
+	return duplicate
+}
+
+// HexMap contains the data for a map instance.
+type HexMap struct {
+	// Xid is the unique ID of the HexMap, used to encourage clients not to blindly interact with a different map.
+	Xid xid.ID `json:"xid"`
+	// Lines is the rough number of rows (in PointyTop orientation) or columns (in FlatTop orientation) in the map.
+	// Because different lines will be staggered, it's somewhat hard to see.
+	Lines uint8 `json:"lines"`
+	// CellsPerLine is the rough number of columns (in PointyTop orientation) or rows (in FlatTop orientation).
+	// This is the number of cells joined together, flat-edge to flat-edge, in each line.
+	CellsPerLine uint8 `json:"cellsPerLine"`
+	// DisplayMode is the orientation and line parity used to display the map.
+	DisplayMode HexMapRepresentation `json:"displayMode"`
+	// LineCells contains the actual map data.
+	// LineCells itself is a slice with Lines elements, each of which is a line;
+	// each of those lines is a slice of CellsPerLine cells.
+	LineCells HexLayer `json:"lineCells"`
+}
+
+// Copy creates a deep copy of this HexMap.
+func (m HexMap) Copy() HexMap {
+	return HexMap{
+		Xid:          m.Xid,
+		Lines:        m.Lines,
+		CellsPerLine: m.CellsPerLine,
+		DisplayMode:  m.DisplayMode,
+		LineCells:    m.LineCells.Copy(),
+	}
+}
diff --git a/server/state/hexorientation.go b/server/state/hexorientation.go
new file mode 100644
index 0000000..60c3449
--- /dev/null
+++ b/server/state/hexorientation.go
@@ -0,0 +1,51 @@
+package state
+
+import "fmt"
+
+// HexOrientation is the enum for the direction hexes are facing.
+type HexOrientation uint8
+
+const (
+	// PointyTop indicates hexes that have a pair of sides on either side in the horizontal direction,
+	// and points on the top and bottom in the vertical direction.
+	PointyTop HexOrientation = 1
+	// FlatTop indicates hexes that have a pair of points on either side in the horizontal direction,
+	// and sides on the top and bottom in the vertical direction.
+	FlatTop HexOrientation = 2
+)
+
+// UnmarshalText unmarshals from the equivalent Javascript constant name.
+func (o *HexOrientation) UnmarshalText(text []byte) error {
+	switch string(text) {
+	case "POINTY_TOP":
+		*o = PointyTop
+		return nil
+	case "FLAT_TOP":
+		*o = FlatTop
+		return nil
+	default:
+		return fmt.Errorf("can't unmarshal unknown HexOrientation %s", text)
+	}
+}
+
+// MarshalText marshals into the equivalent JavaScript constant name.
+func (o HexOrientation) MarshalText() (text []byte, err error) {
+	switch o {
+	case PointyTop, FlatTop:
+		return []byte(o.String()), nil
+	default:
+		return nil, fmt.Errorf("can't marshal unknown HexOrientation %d", o)
+	}
+}
+
+// String returns the equivalent JavaScript constant name.
+func (o HexOrientation) String() string {
+	switch o {
+	case PointyTop:
+		return "POINTY_TOP"
+	case FlatTop:
+		return "FLAT_TOP"
+	default:
+		return fmt.Sprintf("[unknown HexOrientation %d]", o)
+	}
+}
diff --git a/server/state/lineparity.go b/server/state/lineparity.go
new file mode 100644
index 0000000..0f08027
--- /dev/null
+++ b/server/state/lineparity.go
@@ -0,0 +1,49 @@
+package state
+
+import "fmt"
+
+// LineParity indicates whether odd or even lines are indented.
+type LineParity uint8
+
+const (
+	// OddLines indicates that odd lines - 1, 3, 5... - are indented by 1/2 cell.
+	OddLines LineParity = 1
+	// EvenLines indicates that even lines - 0, 2, 4... - are indented by 1/2 cell.
+	EvenLines LineParity = 2
+)
+
+// UnmarshalText unmarshals from the equivalent Javascript constant name.
+func (o *LineParity) UnmarshalText(text []byte) error {
+	switch string(text) {
+	case "ODD":
+		*o = OddLines
+		return nil
+	case "EVEN":
+		*o = EvenLines
+		return nil
+	default:
+		return fmt.Errorf("can't unmarshal unknown LineParity %s", text)
+	}
+}
+
+// MarshalText marshals into the equivalent JavaScript constant name.
+func (o LineParity) MarshalText() (text []byte, err error) {
+	switch o {
+	case OddLines, EvenLines:
+		return []byte(o.String()), nil
+	default:
+		return nil, fmt.Errorf("can't marshal unknown LineParity %d", o)
+	}
+}
+
+// String returns the equivalent JavaScript constant name.
+func (o LineParity) String() string {
+	switch o {
+	case OddLines:
+		return "ODD_LINES"
+	case EvenLines:
+		return "EVEN_LINES"
+	default:
+		return fmt.Sprintf("[unknown LineParity %d]", o)
+	}
+}
diff --git a/server/state/synced.go b/server/state/synced.go
new file mode 100644
index 0000000..5a98c02
--- /dev/null
+++ b/server/state/synced.go
@@ -0,0 +1,15 @@
+package state
+
+// Synced contains all state that is synced between the server and its clients.
+type Synced struct {
+	Map  HexMap   `json:"map"`
+	User UserData `json:"user"`
+}
+
+// Copy creates a deep copy of this Synced instance.
+func (s Synced) Copy() Synced {
+	return Synced{
+		Map:  s.Map.Copy(),
+		User: s.User.Copy(),
+	}
+}
diff --git a/server/state/user.go b/server/state/user.go
new file mode 100644
index 0000000..9d585c4
--- /dev/null
+++ b/server/state/user.go
@@ -0,0 +1,14 @@
+package state
+
+// UserData contains data about clients that is synced between client and server.
+// Unlike the map, UserData is not persisted to disk, and all UserData is lost on shutdown.
+type UserData struct {
+	ActiveColor HexColor `json:"active_color"`
+}
+
+// Copy creates a deep copy of this UserData.
+func (u UserData) Copy() UserData {
+	return UserData{
+		ActiveColor: u.ActiveColor,
+	}
+}