diff --git a/components/config/src/lib.rs b/components/config/src/lib.rs
index 40b4ff1..8643485 100644
--- a/components/config/src/lib.rs
+++ b/components/config/src/lib.rs
@@ -107,6 +107,9 @@ pub struct Config {
     /// Whether to check all external links for validity
     pub check_external_links: bool,
 
+    /// A list of directories containing extra syntax definitions to load.
+    pub extra_syntaxes: Vec<String>,
+
     /// All user params set in [extra] in the config
     pub extra: HashMap<String, Toml>,
 
@@ -233,6 +236,7 @@ impl Default for Config {
             ignored_content: Vec::new(),
             ignored_content_globset: None,
             translations: HashMap::new(),
+            extra_syntaxes: Vec::new(),
             extra: HashMap::new(),
             build_timestamp: Some(1),
         }
diff --git a/components/highlighting/src/lib.rs b/components/highlighting/src/lib.rs
index f814279..2f66cf3 100644
--- a/components/highlighting/src/lib.rs
+++ b/components/highlighting/src/lib.rs
@@ -2,16 +2,20 @@
 extern crate lazy_static;
 extern crate syntect;
 
+use std::cell::RefCell;
+
+use syntect::LoadingError;
 use syntect::dumps::from_binary;
 use syntect::parsing::SyntaxSet;
 use syntect::highlighting::{ThemeSet, Theme};
 use syntect::easy::HighlightLines;
 
 thread_local! {
-    pub static SYNTAX_SET: SyntaxSet = {
+    /// A pair of the set and whether extras have been added to it.
+    pub static SYNTAX_SET: RefCell<(SyntaxSet, bool)> = {
         let mut ss: SyntaxSet = from_binary(include_bytes!("../../../sublime_syntaxes/newlines.packdump"));
         ss.link_syntaxes();
-        ss
+        RefCell::new((ss, false))
     };
 }
 
@@ -19,14 +23,22 @@ lazy_static! {
     pub static ref THEME_SET: ThemeSet = from_binary(include_bytes!("../../../sublime_themes/all.themedump"));
 }
 
+pub fn get_highlighter<'a>(theme: &'a Theme, info: &str, extra_syntaxes: &[String]) -> Result<HighlightLines<'a>, LoadingError> {
+    SYNTAX_SET.with(|rc| {
+        let (ss, extras_added) = &mut *rc.borrow_mut();
+        if !*extras_added {
+            for dir in extra_syntaxes {
+                ss.load_syntaxes(dir, true)?;
+            }
+            ss.link_syntaxes();
+            *extras_added = true;
+        }
 
-pub fn get_highlighter<'a>(theme: &'a Theme, info: &str) -> HighlightLines<'a> {
-    SYNTAX_SET.with(|ss| {
         let syntax = info
             .split(' ')
             .next()
             .and_then(|lang| ss.find_syntax_by_token(lang))
             .unwrap_or_else(|| ss.find_syntax_plain_text());
-        HighlightLines::new(syntax, theme)
+        Ok(HighlightLines::new(syntax, theme))
     })
 }
diff --git a/components/rendering/src/markdown.rs b/components/rendering/src/markdown.rs
index 60ea672..648e3b6 100644
--- a/components/rendering/src/markdown.rs
+++ b/components/rendering/src/markdown.rs
@@ -97,7 +97,13 @@ pub fn markdown_to_html(content: &str, context: &RenderContext) -> Result<(Strin
                     }
 
                     let theme = &THEME_SET.themes[&context.config.highlight_theme];
-                    highlighter = Some(get_highlighter(&theme, info));
+                    match get_highlighter(&theme, info, &context.config.extra_syntaxes) {
+                        Ok(h) => highlighter = Some(h),
+                        Err(err) => {
+                            error = Some(format!("Could not load syntax: {}", err).into());
+                            return Event::Html(Owned(String::new()));
+                        }
+                    }
                     let snippet = start_coloured_html_snippet(theme);
                     Event::Html(Owned(snippet))
                 }