diff --git a/internal/api/api.go b/internal/api/api.go
index 581f3c5..decfde0 100644
--- a/internal/api/api.go
+++ b/internal/api/api.go
@@ -20,7 +20,8 @@ type API struct {
 type Option func(*API)
 
 // WithStore makes the API serve the opportunities in opportunityStore under
-// /opportunities. Without it there are no opportunity routes.
+// /opportunities, and their classifications under /classifications. Without
+// it there are neither.
 func WithStore(opportunityStore *store.Store) Option {
 	return func(a *API) {
 		a.store = opportunityStore
@@ -46,10 +47,12 @@ func New(opts ...Option) *API {
 }
 
 // Routes registers the API's routes on router (mounted under /api): its
-// description (see openapi.go), and the opportunities if it has a store.
+// description (see openapi.go), and the opportunities and classifications if
+// it has a store.
 func (a *API) Routes(router chi.Router) {
 	docsRoutes(router)
 	if a.store != nil {
 		newOpportunityRoutes(a.store, a.events).Routes(router)
+		newClassificationRoutes(a.store).Routes(router)
 	}
 }
diff --git a/internal/api/classifications.go b/internal/api/classifications.go
new file mode 100644
index 0000000..be07f8a
--- /dev/null
+++ b/internal/api/classifications.go
@@ -0,0 +1,54 @@
+package api
+
+import (
+	"net/http"
+
+	"github.com/go-chi/chi/v5"
+
+	"git.avsdev.uk/Go/job-search/internal/httpx"
+	"git.avsdev.uk/Go/job-search/internal/store"
+)
+
+// classificationRoutes wires HTTP handlers to the classification codes in
+// the opportunity store. They're read-only.
+type classificationRoutes struct {
+	store *store.Store
+}
+
+// newClassificationRoutes returns the classification handlers for
+// opportunityStore.
+func newClassificationRoutes(
+	opportunityStore *store.Store,
+) *classificationRoutes {
+	return &classificationRoutes{
+		store: opportunityStore,
+	}
+}
+
+// Routes registers the classification routes on router (mounted under
+// /api).
+func (h *classificationRoutes) Routes(router chi.Router) {
+	router.Get("/classifications", h.list)
+}
+
+// list serves GET /classifications: every classification code that at least
+// one opportunity has, ordered by code.
+//
+//	@Summary		List classifications
+//	@Description	Every classification code (CPV and others) that at least one
+//	@Description	stored opportunity has, once each, ordered by code: the
+//	@Description	codes worth filtering opportunities by.
+//	@Tags			classifications
+//	@Produce		json
+//	@Success		200	{array}		types.ClassificationCode
+//	@Failure		500	{object}	httpx.Problem
+//	@Router			/classifications [get]
+func (h *classificationRoutes) list(w http.ResponseWriter, r *http.Request) {
+	classifications, err := h.store.ListClassifications(r.Context())
+	if err != nil {
+		httpx.WriteProblem(w, r, http.StatusInternalServerError, "")
+		return
+	}
+
+	httpx.WriteJSON(w, http.StatusOK, classifications)
+}
diff --git a/internal/api/classifications_test.go b/internal/api/classifications_test.go
new file mode 100644
index 0000000..8d61ff3
--- /dev/null
+++ b/internal/api/classifications_test.go
@@ -0,0 +1,41 @@
+package api
+
+import (
+	"encoding/json"
+	"net/http"
+	"testing"
+
+	"git.avsdev.uk/Go/job-search/internal/types"
+)
+
+// The classifications the opportunities have are listed once each, by code.
+func TestListClassifications(t *testing.T) {
+	url := opportunitiesServer(t, 3)
+
+	var classifications []types.ClassificationCode
+	status := request(t, "GET", url+"/classifications", &classifications)
+	if status != http.StatusOK {
+		t.Fatalf("status %d", status)
+	}
+	if len(classifications) != 2 ||
+		classifications[0].Code != "03000000" ||
+		classifications[1].Code != "72000000" ||
+		classifications[0].ID == 0 {
+		t.Errorf("got %+v; want 03000000 then 72000000, with ids", classifications)
+	}
+}
+
+// With no opportunities, the list is [] in JSON, not null.
+func TestListClassificationsEmpty(t *testing.T) {
+	url := opportunitiesServer(t, 0)
+	res, err := http.Get(url + "/classifications")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer res.Body.Close()
+	var raw json.RawMessage
+	json.NewDecoder(res.Body).Decode(&raw)
+	if string(raw) != "[]" {
+		t.Errorf("body = %s; want []", raw)
+	}
+}
diff --git a/internal/api/doc.go b/internal/api/doc.go
index 7b4e78b..fb133d6 100644
--- a/internal/api/doc.go
+++ b/internal/api/doc.go
@@ -1,8 +1,8 @@
 // Package api implements the REST API mounted under /api. New assembles it
 // (api.go): the opportunities, read-only apart from their marks
-// (opportunities.go), and the API's own OpenAPI 3.1 description and docs
-// viewer (openapi.go). New resources are added in New and Routes, so the
-// server only mounts the API.
+// (opportunities.go), their classifications (classifications.go), and the
+// API's own OpenAPI 3.1 description and docs viewer (openapi.go). New
+// resources are added in New and Routes, so the server only mounts the API.
 //
 // The description is generated from the swag annotations on this package's
 // handlers by go generate (swag v2, see the README's dev tools), into
diff --git a/internal/api/opportunities_test.go b/internal/api/opportunities_test.go
index 6a2da56..2c8cc3b 100644
--- a/internal/api/opportunities_test.go
+++ b/internal/api/opportunities_test.go
@@ -42,6 +42,15 @@ func opportunitiesServer(t *testing.T, count int, opts ...Option) string {
 			UUIDType: "ocid",
 			Title:    fmt.Sprintf("Opportunity %d", i),
 			Source:   "FindATender",
+			// Two codes, shared: ocds-1, ocds-3... have 72000000, the
+			// others 03000000.
+			Classifications: []types.ClassificationCode{
+				{
+					IsCPV:          true,
+					Code:           []string{"03000000", "72000000"}[i%2],
+					Classification: []string{"Agriculture", "IT services"}[i%2],
+				},
+			},
 			Releases: []types.OpportunityRelease{
 				{
 					ReleaseID:   "r1",
diff --git a/internal/store/queries.go b/internal/store/queries.go
index ff32c2a..5b56654 100644
--- a/internal/store/queries.go
+++ b/internal/store/queries.go
@@ -75,7 +75,9 @@ SELECT t.* FROM opportunities t WHERE t.uuid = ?`
 
 // ListOpportunities is assembled by listOpportunitiesQuery as:
 //
-//	listOpportunitiesSelect [WHERE filter AND filter ...] listOpportunitiesOrderAndPage
+//	listOpportunitiesSelect
+//	[WHERE filter AND filter ...]
+//	listOpportunitiesOrderAndPage
 //
 // Each filterOpportunity* fragment takes exactly one "?" argument, except
 // filterOpportunityMark* fragments, which take none. A mark fragment is true
@@ -213,6 +215,17 @@ INSERT INTO opportunity_classifications (opportunity_uuid, classification_id)
 VALUES (?, ?)
 ON CONFLICT DO NOTHING`
 
+// selectClassificationsInUse reads every classification code that at least
+// one opportunity has, once each. c.* must map entirely onto
+// types.ClassificationCode.
+const selectClassificationsInUse = `
+SELECT c.* FROM classification_codes c
+WHERE EXISTS (
+	SELECT 1 FROM opportunity_classifications tc
+	WHERE tc.classification_id = c.id
+)
+ORDER BY c.code, c.id`
+
 // selectClassificationsForOpportunities reads the classifications of several
 // opportunities. "IN (?)" is expanded by sqlx.In, which must be followed by
 // Rebind. c.* must map entirely onto types.ClassificationCode.
diff --git a/internal/store/queries_test.go b/internal/store/queries_test.go
index ab4421b..00fc38f 100644
--- a/internal/store/queries_test.go
+++ b/internal/store/queries_test.go
@@ -100,6 +100,9 @@ var checks = map[string]queryCheck{
 	"insertOpportunityClassification": {
 		sql: insertOpportunityClassification,
 	},
+	"selectClassificationsInUse": {
+		sql: selectClassificationsInUse,
+	},
 	"selectClassificationsForOpportunities": {
 		sql: selectClassificationsForOpportunities,
 		in:  true,
diff --git a/internal/store/store.go b/internal/store/store.go
index b7b57b1..6a24dfd 100644
--- a/internal/store/store.go
+++ b/internal/store/store.go
@@ -717,6 +717,21 @@ func (s *Store) ListReleases(
 	return releases, nil
 }
 
+// ListClassifications returns every classification code that at least one
+// stored opportunity has, ordered by code. A code no opportunity has any more
+// isn't listed. The result is never nil.
+func (s *Store) ListClassifications(
+	ctx context.Context,
+) ([]types.ClassificationCode, error) {
+	classifications := []types.ClassificationCode{}
+	err := s.db.SelectContext(ctx, &classifications, selectClassificationsInUse)
+	if err != nil {
+		return nil, fmt.Errorf("listing classifications: %w", err)
+	}
+
+	return classifications, nil
+}
+
 // LatestReleaseDate returns the date of the newest release stored for the
 // opportunities from source, or the zero time if none are stored, so a
 // fetcher can carry on from where it left off.
diff --git a/internal/store/store_test.go b/internal/store/store_test.go
index 831918c..5c57d9e 100644
--- a/internal/store/store_test.go
+++ b/internal/store/store_test.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -902,3 +903,44 @@ func TestSaveResultNewOpportunity(t *testing.T) {
 		)
 	}
 }
+
+// ListClassifications lists the codes in use, once each and sorted, and not
+// a code no opportunity has any more.
+func TestListClassifications(t *testing.T) {
+	s := openStore(t)
+	ctx := context.Background()
+
+	got, err := s.ListClassifications(ctx)
+	if err != nil || got == nil || len(got) != 0 {
+		t.Fatalf("empty store: %v, %v; want an empty, non-nil list", got, err)
+	}
+
+	mustSave(t, s, opportunity("ocds-1", "First", 1, "r1"))
+	mustSave(t, s, opportunity("ocds-2", "Second", 1, "r1"))
+	codes := func() []string {
+		t.Helper()
+		got, err := s.ListClassifications(ctx)
+		if err != nil {
+			t.Fatal(err)
+		}
+		var listed []string
+		for _, classification := range got {
+			listed = append(listed, classification.Code)
+		}
+		return listed
+	}
+	if got := strings.Join(codes(), ","); got != "03000000,72000000" {
+		t.Fatalf("codes = %s; want 03000000,72000000", got)
+	}
+
+	// Newer releases replace both opportunities' classifications with one
+	// code; the other is still stored, but no longer listed.
+	for _, uuid := range []string{"ocds-1", "ocds-2"} {
+		newer := opportunity(uuid, "Newer", 2, "r2")
+		newer.Classifications = newer.Classifications[1:]
+		mustSave(t, s, newer)
+	}
+	if got := strings.Join(codes(), ","); got != "03000000" {
+		t.Errorf("after the change: codes = %s; want 03000000", got)
+	}
+}