diff --git a/models/categories.go b/models/categories.go
new file mode 100644
index 0000000..5655009
--- /dev/null
+++ b/models/categories.go
@@ -0,0 +1,61 @@
+package models
+
+import (
+	"strings"
+	"time"
+	"unicode"
+)
+
+type Category struct {
+	ID          int64     `json:"id"`
+	SiteID      int64     `json:"site_id"`
+	GUID        string    `json:"guid"`
+	Name        string    `json:"name"`
+	Slug        string    `json:"slug"`
+	Description string    `json:"description"`
+	CreatedAt   time.Time `json:"created_at"`
+	UpdatedAt   time.Time `json:"updated_at"`
+}
+
+// CategoryWithCount is a Category plus the count of published posts in it.
+type CategoryWithCount struct {
+	Category
+	PostCount        int
+	DescriptionBrief string
+}
+
+// GenerateCategorySlug creates a URL-safe slug from a category name.
+// e.g. "Go Programming" -> "go-programming"
+func GenerateCategorySlug(name string) string {
+	var sb strings.Builder
+	prevDash := false
+	for _, c := range strings.TrimSpace(name) {
+		if unicode.IsLetter(c) || unicode.IsNumber(c) {
+			sb.WriteRune(unicode.ToLower(c))
+			prevDash = false
+		} else if unicode.IsSpace(c) || c == '-' || c == '_' {
+			if !prevDash && sb.Len() > 0 {
+				sb.WriteRune('-')
+				prevDash = true
+			}
+		}
+	}
+	result := sb.String()
+	return strings.TrimRight(result, "-")
+}
+
+// BriefDescription returns the first sentence or line of the description.
+func BriefDescription(desc string) string {
+	if desc == "" {
+		return ""
+	}
+	for i, c := range desc {
+		if c == '\n' {
+			return desc[:i]
+		}
+		if c == '.' && i+1 < len(desc) {
+			return desc[:i+1]
+		}
+	}
+	return desc
+}
diff --git a/models/categories_test.go b/models/categories_test.go
new file mode 100644
index 0000000..facf08b
--- /dev/null
+++ b/models/categories_test.go
@@ -0,0 +1,28 @@
+package models_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"lmika.dev/lmika/weiro/models"
+)
+
+func TestGenerateCategorySlug(t *testing.T) {
+	tests := []struct {
+		name string
+		want string
+	}{
+		{"Go Programming", "go-programming"},
+		{"  Travel  ", "travel"},
+		{"hello---world", "hello-world"},
+		{"UPPER CASE", "upper-case"},
+		{"one", "one"},
+		{"with_underscores", "with-underscores"},
+		{"special!@#chars", "specialchars"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.want, models.GenerateCategorySlug(tt.name))
+		})
+	}
+}
diff --git a/models/errors.go b/models/errors.go
index 997a952..eda780c 100644
--- a/models/errors.go
+++ b/models/errors.go
@@ -7,3 +7,4 @@ var PermissionError = errors.New("permission denied")
 var NotFoundError = errors.New("not found")
 var SiteRequiredError = errors.New("site required")
 var DeleteDebounceError = errors.New("permanent delete too soon, try again in a few seconds")
+var SlugConflictError = errors.New("a category with this slug already exists")
diff --git a/sql/schema/04_categories.up.sql b/sql/schema/04_categories.up.sql
new file mode 100644
index 0000000..260d06b
--- /dev/null
+++ b/sql/schema/04_categories.up.sql
@@ -0,0 +1,23 @@
+CREATE TABLE categories (
+    id          INTEGER PRIMARY KEY AUTOINCREMENT,
+    site_id     INTEGER NOT NULL,
+    guid        TEXT    NOT NULL,
+    name        TEXT    NOT NULL,
+    slug        TEXT    NOT NULL,
+    description TEXT    NOT NULL DEFAULT '',
+    created_at  INTEGER NOT NULL,
+    updated_at  INTEGER NOT NULL,
+    FOREIGN KEY (site_id) REFERENCES sites (id) ON DELETE CASCADE
+);
+CREATE INDEX idx_categories_site ON categories (site_id);
+CREATE UNIQUE INDEX idx_categories_guid ON categories (guid);
+CREATE UNIQUE INDEX idx_categories_site_slug ON categories (site_id, slug);
+
+CREATE TABLE post_categories (
+    post_id     INTEGER NOT NULL,
+    category_id INTEGER NOT NULL,
+    PRIMARY KEY (post_id, category_id),
+    FOREIGN KEY (post_id) REFERENCES posts (id) ON DELETE CASCADE,
+    FOREIGN KEY (category_id) REFERENCES categories (id) ON DELETE CASCADE
+);
+CREATE INDEX idx_post_categories_category ON post_categories (category_id);