diff --git a/examples/nop-preprocessor.rs b/examples/nop-preprocessor.rs
new file mode 100644
index 00000000..1349a591
--- /dev/null
+++ b/examples/nop-preprocessor.rs
@@ -0,0 +1,40 @@
+extern crate mdbook;
+#[macro_use]
+extern crate clap;
+
+use clap::{App, Arg, SubCommand, ArgMatches};
+use mdbook::preprocess::{Preprocessor, PreprocessorContext};
+
+fn main() {
+    let matches = app().get_matches();
+
+    if let Some(sub_args) = matches.subcommand_matches("supports") {
+        handle_supports(sub_args);
+    } else {
+        handle_preprocessing(&matches);
+    }
+}
+
+fn handle_preprocessing(args: &ArgMatches) {
+
+}
+
+fn handle_supports(sub_args: &ArgMatches) {
+        let renderer = sub_args.value_of("renderer").expect("Required argument");
+        let supported = renderer_is_supported(&renderer);
+}
+
+fn renderer_is_supported(renderer: &str) -> bool {
+    true
+}
+
+fn app() -> App<'static, 'static> {
+    app_from_crate!().subcommand(
+        SubCommand::with_name("supports")
+            .arg(Arg::with_name("renderer").required(true))
+            .about(
+                "Check whether a renderer is supported by this preprocessor",
+            ),
+    )
+}
+
diff --git a/src/preprocess/cmd.rs b/src/preprocess/cmd.rs
new file mode 100644
index 00000000..2047cc43
--- /dev/null
+++ b/src/preprocess/cmd.rs
@@ -0,0 +1,111 @@
+use super::{Preprocessor, PreprocessorContext};
+use book::Book;
+use errors::*;
+use shlex::Shlex;
+use std::io::{self, Read};
+use serde_json;
+use std::process::{Command, Stdio, Child};
+
+/// A custom preprocessor which will shell out to a 3rd-party program.
+///
+/// # Preprocessing
+///
+/// When the `supports_renderer()` method is executed, `CmdPreprocessor` will
+/// execute the shell command `$cmd supports $renderer`. If the renderer is
+/// supported, custom preprocessors should exit with a exit code of `0`,
+/// any other exit code be considered as unsupported.
+#[derive(Debug, Clone, PartialEq)]
+pub struct CmdPreprocessor {
+    name: String,
+    cmd: String,
+}
+
+impl CmdPreprocessor {
+    /// Create a new `CmdPreprocessor`.
+    pub fn new(name: String, cmd: String) -> CmdPreprocessor {
+        CmdPreprocessor { name, cmd }
+    }
+
+    /// A convenience function custom preprocessors can use to parse the input
+    /// written to `stdin` by a `CmdRenderer`.
+    pub fn parse_input<R: Read>(
+        reader: R,
+    ) -> Result<(PreprocessorContext, Book)> {
+        serde_json::from_reader(reader).chain_err(|| "Unable to parse the input")
+    }
+
+    fn write_input(&self, child: &mut Child, book: Book, ctx: PreprocessorContext) {
+            let mut stdin = child.stdin.take().expect("Child has stdin");
+            let input = (ctx, book);
+
+            if let Err(e) = serde_json::to_writer(&mut stdin, &input) {
+                // Looks like the backend hung up before we could finish
+                // sending it the render context. Log the error and keep going
+                warn!("Error writing the RenderContext to the backend, {}", e);
+            }
+
+            // explicitly close the `stdin` file handle
+            drop(stdin);
+    }
+
+    fn command(&self) -> Result<Command> {
+        let mut words = Shlex::new(&self.cmd);
+        let executable = match words.next() {
+            Some(e) => e,
+            None => bail!("Command string was empty"),
+        };
+
+        let mut cmd = Command::new(executable);
+
+        for arg in words {
+            cmd.arg(arg);
+        }
+
+        Ok(cmd)
+    }
+}
+
+impl Preprocessor for CmdPreprocessor {
+    fn name(&self) -> &str {
+        &self.name
+    }
+
+    fn run(&self, ctx: &PreprocessorContext, book: Book) -> Result<Book> {
+        unimplemented!()
+    }
+
+    fn supports_renderer(&self, renderer: &str) -> bool {
+        let mut cmd = match self.command() {
+            Ok(c) => c,
+            Err(e) => {
+                warn!("Unable to create the command for the \"{}\" preprocessor, {}", self.name(), e);
+                return true;
+            }
+        };
+
+        cmd
+            .arg("supports")
+            .arg(renderer)
+            .stdin(Stdio::piped())
+            .stdout(Stdio::inherit())
+            .stderr(Stdio::inherit());
+
+        let child = match cmd.spawn() {
+            Ok(c) => c,
+            Err(e) => {
+                if e.kind() == io::ErrorKind::NotFound {
+                    warn!(
+                    "The command wasn't found, is the \"{}\" preprocessor installed?",
+                    self.name
+                );
+                    warn!("\tCommand: {}", self.cmd);
+                }
+
+                // give it the benefit of the doubt
+                return true;
+            }
+        };
+
+        unimplemented!()
+    }
+}
diff --git a/src/preprocess/mod.rs b/src/preprocess/mod.rs
index 87a135db..b7ab1986 100644
--- a/src/preprocess/mod.rs
+++ b/src/preprocess/mod.rs
@@ -2,9 +2,11 @@
 
 pub use self::index::IndexPreprocessor;
 pub use self::links::LinkPreprocessor;
+pub use self::cmd::CmdPreprocessor;
 
 mod index;
 mod links;
+mod cmd;
 
 use book::Book;
 use config::Config;
@@ -14,6 +16,7 @@ use std::path::PathBuf;
 
 /// Extra information for a `Preprocessor` to give them more context when
 /// processing a book.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct PreprocessorContext {
     /// The location of the book directory on disk.
     pub root: PathBuf,
diff --git a/tests/custom_preprocessors.rs b/tests/custom_preprocessors.rs
new file mode 100644
index 00000000..bad64f98
--- /dev/null
+++ b/tests/custom_preprocessors.rs
@@ -0,0 +1,16 @@
+extern crate mdbook;
+
+use mdbook::preprocess::{CmdPreprocessor, Preprocessor};
+
+fn example() -> CmdPreprocessor {
+    CmdPreprocessor::new("nop-preprocessor".to_string(), "cargo run --example nop-preprocessor --".to_string())
+}
+
+#[test]
+fn check_if_renderer_is_supported() {
+    let cmd = example();
+
+    let got = cmd.supports_renderer("whatever");
+
+    assert_eq!(got, true);
+}