diff --git a/src/cargo/core/manifest.rs b/src/cargo/core/manifest.rs
index 204d55a1ea6..d03cf688b93 100644
--- a/src/cargo/core/manifest.rs
+++ b/src/cargo/core/manifest.rs
@@ -47,6 +47,7 @@ pub struct VirtualManifest {
 pub struct ManifestMetadata {
     pub authors: Vec<String>,
     pub keywords: Vec<String>,
+    pub categories: Vec<String>,
     pub license: Option<String>,
     pub license_file: Option<String>,
     pub description: Option<String>,    // not markdown
diff --git a/src/cargo/ops/registry.rs b/src/cargo/ops/registry.rs
index 58b2894cd8b..2397f8204a9 100644
--- a/src/cargo/ops/registry.rs
+++ b/src/cargo/ops/registry.rs
@@ -113,6 +113,7 @@ fn transmit(config: &Config,
     let ManifestMetadata {
         ref authors, ref description, ref homepage, ref documentation,
         ref keywords, ref readme, ref repository, ref license, ref license_file,
+        ref categories,
     } = *manifest.metadata();
     let readme = match *readme {
         Some(ref readme) => Some(paths::read(&pkg.root().join(readme))?),
@@ -133,7 +134,7 @@ fn transmit(config: &Config,
         return Ok(());
     }
 
-    registry.publish(&NewCrate {
+    let publish = registry.publish(&NewCrate {
         name: pkg.name().to_string(),
         vers: pkg.version().to_string(),
         deps: deps,
@@ -143,13 +144,27 @@ fn transmit(config: &Config,
         homepage: homepage.clone(),
         documentation: documentation.clone(),
         keywords: keywords.clone(),
+        categories: categories.clone(),
         readme: readme,
         repository: repository.clone(),
         license: license.clone(),
         license_file: license_file.clone(),
-    }, tarball).map_err(|e| {
-        human(e.to_string())
-    })
+    }, tarball);
+
+    match publish {
+        Ok(warnings) => {
+            if !warnings.invalid_categories.is_empty() {
+                let msg = format!("\
+                    the following are not valid category slugs and were \
+                    ignored: {}. Please see https://crates.io/category_slugs \
+                    for the list of all category slugs. \
+                    ", warnings.invalid_categories.join(", "));
+                config.shell().warn(&msg)?;
+            }
+            Ok(())
+        },
+        Err(e) => Err(human(e.to_string())),
+    }
 }
 
 pub fn registry_configuration(config: &Config) -> CargoResult<RegistryConfig> {
diff --git a/src/cargo/util/toml.rs b/src/cargo/util/toml.rs
index 3bcbe7c9550..8abbaa2f183 100644
--- a/src/cargo/util/toml.rs
+++ b/src/cargo/util/toml.rs
@@ -317,6 +317,7 @@ pub struct TomlProject {
     documentation: Option<String>,
     readme: Option<String>,
     keywords: Option<Vec<String>>,
+    categories: Option<Vec<String>>,
     license: Option<String>,
     license_file: Option<String>,
     repository: Option<String>,
@@ -654,6 +655,7 @@ impl TomlManifest {
             license_file: project.license_file.clone(),
             repository: project.repository.clone(),
             keywords: project.keywords.clone().unwrap_or(Vec::new()),
+            categories: project.categories.clone().unwrap_or(Vec::new()),
         };
 
         let workspace_config = match (self.workspace.as_ref(),
diff --git a/src/crates-io/lib.rs b/src/crates-io/lib.rs
index 884460df290..00637be3dc2 100644
--- a/src/crates-io/lib.rs
+++ b/src/crates-io/lib.rs
@@ -10,7 +10,7 @@ use std::io::{self, Cursor};
 use std::result;
 
 use curl::easy::{Easy, List};
-use rustc_serialize::json;
+use rustc_serialize::json::{self, Json};
 
 use url::percent_encoding::{percent_encode, QUERY_ENCODE_SET};
 
@@ -39,6 +39,7 @@ pub enum Error {
     NotFound,
     JsonEncodeError(json::EncoderError),
     JsonDecodeError(json::DecoderError),
+    JsonParseError(json::ParserError),
 }
 
 impl From<json::EncoderError> for Error {
@@ -53,6 +54,12 @@ impl From<json::DecoderError> for Error {
     }
 }
 
+impl From<json::ParserError> for Error {
+    fn from(err: json::ParserError) -> Error {
+        Error::JsonParseError(err)
+    }
+}
+
 impl From<curl::Error> for Error {
     fn from(err: curl::Error) -> Error {
         Error::Curl(err)
@@ -78,6 +85,7 @@ pub struct NewCrate {
     pub homepage: Option<String>,
     pub readme: Option<String>,
     pub keywords: Vec<String>,
+    pub categories: Vec<String>,
     pub license: Option<String>,
     pub license_file: Option<String>,
     pub repository: Option<String>,
@@ -103,6 +111,10 @@ pub struct User {
     pub name: Option<String>,
 }
 
+pub struct Warnings {
+    pub invalid_categories: Vec<String>,
+}
+
 #[derive(RustcDecodable)] struct R { ok: bool }
 #[derive(RustcDecodable)] struct ApiErrorList { errors: Vec<ApiError> }
 #[derive(RustcDecodable)] struct ApiError { detail: String }
@@ -110,7 +122,6 @@ pub struct User {
 #[derive(RustcDecodable)] struct Users { users: Vec<User> }
 #[derive(RustcDecodable)] struct TotalCrates { total: u32 }
 #[derive(RustcDecodable)] struct Crates { crates: Vec<Crate>, meta: TotalCrates }
-
 impl Registry {
     pub fn new(host: String, token: Option<String>) -> Registry {
         Registry::new_handle(host, token, Easy::new())
@@ -147,7 +158,8 @@ impl Registry {
         Ok(json::decode::<Users>(&body)?.users)
     }
 
-    pub fn publish(&mut self, krate: &NewCrate, tarball: &File) -> Result<()> {
+    pub fn publish(&mut self, krate: &NewCrate, tarball: &File)
+                   -> Result<Warnings> {
         let json = json::encode(krate)?;
         // Prepare the body. The format of the upload request is:
         //
@@ -190,10 +202,24 @@ impl Registry {
         headers.append(&format!("Authorization: {}", token))?;
         self.handle.http_headers(headers)?;
 
-        let _body = handle(&mut self.handle, &mut |buf| {
+        let body = handle(&mut self.handle, &mut |buf| {
             body.read(buf).unwrap_or(0)
         })?;
-        Ok(())
+        // Can't derive RustcDecodable because JSON has a key named "crate" :(
+        let response = if body.len() > 0 {
+            Json::from_str(&body)?
+        } else {
+            Json::from_str("{}")?
+        };
+        let invalid_categories: Vec<String> =
+            response
+                .find_path(&["warnings", "invalid_categories"])
+                .and_then(Json::as_array)
+                .map(|x| {
+                    x.iter().flat_map(Json::as_string).map(Into::into).collect()
+                })
+                .unwrap_or_else(Vec::new);
+        Ok(Warnings { invalid_categories: invalid_categories })
     }
 
     pub fn search(&mut self, query: &str, limit: u8) -> Result<(Vec<Crate>, u32)> {
@@ -328,6 +354,7 @@ impl fmt::Display for Error {
             Error::NotFound => write!(f, "cannot find crate"),
             Error::JsonEncodeError(ref e) => write!(f, "json encode error: {}", e),
             Error::JsonDecodeError(ref e) => write!(f, "json decode error: {}", e),
+            Error::JsonParseError(ref e) => write!(f, "json parse error: {}", e),
         }
     }
 }
diff --git a/src/doc/manifest.md b/src/doc/manifest.md
index ccdc6c3cc67..09fa5e04e9f 100644
--- a/src/doc/manifest.md
+++ b/src/doc/manifest.md
@@ -118,10 +118,16 @@ repository = "..."
 # contents of this file are stored and indexed in the registry.
 readme = "..."
 
-# This is a small list of keywords used to categorize and search for this
-# package.
+# This is a list of up to five keywords that describe this crate. Keywords
+# are searchable on crates.io, and you may choose any words that would
+# help someone find this crate.
 keywords = ["...", "..."]
 
+# This is a list of up to five categories where this crate would fit.
+# Categories are a fixed list available at crates.io/categories, and
+# they must match exactly.
+categories = ["...", "..."]
+
 # This is a string description of the license for this package. Currently
 # crates.io will validate the license provided against a whitelist of known
 # license identifiers from http://spdx.org/licenses/. Multiple licenses can be