diff --git a/NAMESPACE b/NAMESPACE
index b0f48df..1550d74 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -1,5 +1,6 @@
 # Generated by roxygen2: do not edit by hand
 
+export(cog_categories)
 export(cog_explain)
 export(cog_find_peers)
 export(cog_geographic_rollup)
diff --git a/R/categories.R b/R/categories.R
new file mode 100644
index 0000000..d1903e1
--- /dev/null
+++ b/R/categories.R
@@ -0,0 +1,60 @@
+# R/categories.R
+
+#' List available spending / revenue categories
+#'
+#' Returns the category taxonomy exposed by the corpus's
+#' `summary_categories` view, grouped to one row per
+#' `(category, subtype)` pair. Use this to discover valid `category`
+#' values for [cog_spending()] / [cog_revenue()] /
+#' [cog_geographic_rollup()] and to audit which Census item codes feed
+#' each category.
+#'
+#' @param type Either `NULL` (default, return both spending and revenue
+#'   rows), `"spending"`, or `"revenue"`.
+#' @param pattern Optional regex matched case-insensitively against the
+#'   `category` column (e.g. `"Police"` or `"Tax"`).
+#' @return Tibble with columns `category`, `category_type`, `subtype`,
+#'   `n_codes`, `item_codes` (comma-separated, alphabetical). Sorted by
+#'   `category_type`, `category`, `subtype`.
+#' @export
+cog_categories <- function(type = NULL, pattern = NULL) {
+  if (!is.null(type)) {
+    if (!is.character(type) || length(type) != 1L ||
+        !type %in% c("spending", "revenue")) {
+      cli::cli_abort('`type` must be NULL, "spending", or "revenue".')
+    }
+  }
+  if (!is.null(pattern) &&
+      (!is.character(pattern) || length(pattern) != 1L)) {
+    cli::cli_abort("`pattern` must be a length-1 character string or NULL.")
+  }
+
+  con <- .ensure_session()
+
+  preds <- character(0)
+  if (!is.null(type)) {
+    # Translate user-facing "spending" to the corpus's native "expenditure"
+    # value so callers don't have to learn Census vocabulary. "revenue" is
+    # the same in both.
+    db_type <- if (type == "spending") "expenditure" else type
+    preds <- c(preds, sprintf("category_type = %s", .sql_lit_chr(db_type)))
+  }
+  if (!is.null(pattern)) {
+    preds <- c(preds,
+               sprintf("regexp_matches(category, %s, 'i')",
+                       .sql_lit_chr(pattern)))
+  }
+  where <- if (length(preds) == 0L) "" else paste("WHERE", paste(preds, collapse = " AND "))
+
+  sql <- paste(
+    "SELECT category, category_type,
+            COALESCE(spend_subtype, revenue_subtype) AS subtype,
+            COUNT(DISTINCT item_code) AS n_codes,
+            string_agg(DISTINCT item_code, ',' ORDER BY item_code) AS item_codes
+     FROM summary_categories",
+    where,
+    "GROUP BY category, category_type, subtype
+     ORDER BY category_type, category, subtype"
+  )
+  tibble::as_tibble(DBI::dbGetQuery(con, sql))
+}
diff --git a/R/session.R b/R/session.R
index b385097..fc83819 100644
--- a/R/session.R
+++ b/R/session.R
@@ -33,13 +33,13 @@ cog_open <- function(url = .resolve_url(),
   .uscogdata_env$con
 }
 
+# Coerce an input to a character vector of canonical_govid values.
+# Accepts either a character vector (returned as-is after `as.character`)
+# or a data.frame / tibble with a `canonical_govid` column (such as the
+# output of cog_gov_search() or cog_find_peers()) — in that case the
+# column is extracted so results from discovery verbs can pipe directly
+# into the query verbs.
 #' @noRd
-#' Coerce an input to a character vector of canonical_govid values.
-#' Accepts either a character vector (returned as-is after `as.character`)
-#' or a data.frame / tibble with a `canonical_govid` column (such as the
-#' output of [cog_gov_search()] or [cog_find_peers()]) — in that case the
-#' column is extracted so results from discovery verbs can pipe directly
-#' into the query verbs.
 .coerce_govid_input <- function(x, arg = "govid") {
   if (is.data.frame(x)) {
     if (!"canonical_govid" %in% names(x)) {
@@ -58,11 +58,11 @@ cog_open <- function(url = .resolve_url(),
   as.character(x)
 }
 
+# Check which of the supplied govids exist in canonical_fips_xwalk.
+# Emits a cli message listing any missing ones alongside a pointer to the
+# v0.1 scope explanation; returns both sets so callers can attach them to
+# provenance.
 #' @noRd
-#' Check which of the supplied govids exist in canonical_fips_xwalk.
-#' Emits a cli message listing any missing ones alongside a pointer to the
-#' v0.1 scope explanation; returns both sets so callers can attach them to
-#' provenance.
 .check_govids_in_scope <- function(govids) {
   govids <- unique(as.character(govids))
   if (length(govids) == 0L) return(list(found = character(0), missing = character(0)))
diff --git a/man/cog_categories.Rd b/man/cog_categories.Rd
new file mode 100644
index 0000000..6cef86a
--- /dev/null
+++ b/man/cog_categories.Rd
@@ -0,0 +1,28 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/categories.R
+\name{cog_categories}
+\alias{cog_categories}
+\title{List available spending / revenue categories}
+\usage{
+cog_categories(type = NULL, pattern = NULL)
+}
+\arguments{
+\item{type}{Either `NULL` (default, return both spending and revenue
+rows), `"spending"`, or `"revenue"`.}
+
+\item{pattern}{Optional regex matched case-insensitively against the
+`category` column (e.g. `"Police"` or `"Tax"`).}
+}
+\value{
+Tibble with columns `category`, `category_type`, `subtype`,
+  `n_codes`, `item_codes` (comma-separated, alphabetical). Sorted by
+  `category_type`, `category`, `subtype`.
+}
+\description{
+Returns the category taxonomy exposed by the corpus's
+`summary_categories` view, grouped to one row per
+`(category, subtype)` pair. Use this to discover valid `category`
+values for [cog_spending()] / [cog_revenue()] /
+[cog_geographic_rollup()] and to audit which Census item codes feed
+each category.
+}
diff --git a/tests/testthat/test-categories.R b/tests/testthat/test-categories.R
new file mode 100644
index 0000000..af802f8
--- /dev/null
+++ b/tests/testthat/test-categories.R
@@ -0,0 +1,62 @@
+test_that("cog_categories returns all categories grouped by subtype", {
+  skip_if_no_corpus()
+  r <- cog_categories()
+  expect_s3_class(r, "tbl_df")
+  expected <- c("category", "category_type", "subtype",
+                "n_codes", "item_codes")
+  expect_true(all(expected %in% names(r)))
+  expect_gt(nrow(r), 10L)
+  # corpus preserves Census-native "expenditure" vocabulary; the API takes
+  # "spending" as a friendlier alias.
+  expect_setequal(unique(r$category_type), c("expenditure", "revenue"))
+})
+
+test_that("cog_categories(type = 'spending') returns only expenditure rows", {
+  skip_if_no_corpus()
+  r <- cog_categories(type = "spending")
+  expect_true(all(r$category_type == "expenditure"))
+  expect_true(all(r$subtype %in% c("operations", "capital")))
+})
+
+test_that("cog_categories(type = 'revenue') returns only revenue rows", {
+  skip_if_no_corpus()
+  r <- cog_categories(type = "revenue")
+  expect_true(all(r$category_type == "revenue"))
+  expect_true(all(r$subtype %in%
+                  c("own_source", "federal", "state", "local_aid")))
+})
+
+test_that("cog_categories(pattern = ...) filters case-insensitively", {
+  skip_if_no_corpus()
+  r <- cog_categories(pattern = "police")
+  expect_gt(nrow(r), 0L)
+  expect_true(all(grepl("Police", r$category, ignore.case = TRUE)))
+})
+
+test_that("cog_categories has one row per (category, subtype)", {
+  skip_if_no_corpus()
+  r <- cog_categories()
+  key <- paste(r$category, r$subtype, sep = "|")
+  expect_equal(length(key), length(unique(key)))
+})
+
+test_that("cog_categories item_codes is non-empty comma-separated string", {
+  skip_if_no_corpus()
+  r <- cog_categories()
+  expect_true(all(nzchar(r$item_codes)))
+  expect_true(all(r$n_codes >= 1L))
+  # n_codes should equal count of commas + 1
+  expect_equal(r$n_codes,
+               vapply(strsplit(r$item_codes, ","), length, integer(1)))
+})
+
+test_that("cog_categories sorted by category_type, category, subtype", {
+  skip_if_no_corpus()
+  r <- cog_categories()
+  sorted <- r[order(r$category_type, r$category, r$subtype), ]
+  expect_identical(r, sorted)
+})
+
+test_that("cog_categories rejects invalid type", {
+  expect_error(cog_categories(type = "both"), "type")
+})