diff --git a/src/format.rs b/src/format.rs
new file mode 100644
index 0000000..f72ed39
--- /dev/null
+++ b/src/format.rs
@@ -0,0 +1,114 @@
+use std::fmt;
+use std::cell::RefCell;
+
+/// Format all iterator elements lazily, separated by `sep`.
+///
+/// The format value can only be formatted once, after that the iterator is
+/// exhausted.
+///
+/// See [`.format_with()`](../trait.Itertools.html#method.format_with) for more information.
+#[derive(Clone)]
+pub struct FormatWith<'a, I, F> {
+    sep: &'a str,
+    /// FormatWith uses interior mutability because Display::fmt takes &self.
+    inner: RefCell<Option<(I, F)>>,
+}
+
+/// Format all iterator elements lazily, separated by `sep`.
+///
+/// The format value can only be formatted once, after that the iterator is
+/// exhausted.
+///
+/// See [`.format()`](../trait.Itertools.html#method.format)
+/// for more information.
+#[derive(Clone)]
+pub struct Format<'a, I> {
+    sep: &'a str,
+    /// Format uses interior mutability because Display::fmt takes &self.
+    inner: RefCell<Option<I>>,
+}
+
+pub fn new_format<'a, I, F>(iter: I, separator: &'a str, f: F) -> FormatWith<'a, I, F>
+    where I: Iterator,
+          F: FnMut(I::Item, &mut dyn FnMut(&dyn fmt::Display) -> fmt::Result) -> fmt::Result
+{
+    FormatWith {
+        sep: separator,
+        inner: RefCell::new(Some((iter, f))),
+    }
+}
+
+pub fn new_format_default<'a, I>(iter: I, separator: &'a str) -> Format<'a, I>
+    where I: Iterator,
+{
+    Format {
+        sep: separator,
+        inner: RefCell::new(Some(iter)),
+    }
+}
+
+impl<'a, I, F> fmt::Display for FormatWith<'a, I, F>
+    where I: Iterator,
+          F: FnMut(I::Item, &mut dyn  FnMut(&dyn fmt::Display) -> fmt::Result) -> fmt::Result
+{
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let (mut iter, mut format) = match self.inner.borrow_mut().take() {
+            Some(t) => t,
+            None => panic!("FormatWith: was already formatted once"),
+        };
+
+        if let Some(fst) = iter.next() {
+            format(fst, &mut |disp: &dyn fmt::Display| disp.fmt(f))?;
+            for elt in iter {
+                if self.sep.len() > 0 {
+
+                    f.write_str(self.sep)?;
+                }
+                format(elt, &mut |disp: &dyn fmt::Display| disp.fmt(f))?;
+            }
+        }
+        Ok(())
+    }
+}
+
+impl<'a, I> Format<'a, I>
+    where I: Iterator,
+{
+    fn format<F>(&self, f: &mut fmt::Formatter, mut cb: F) -> fmt::Result
+        where F: FnMut(&I::Item, &mut fmt::Formatter) -> fmt::Result,
+    {
+        let mut iter = match self.inner.borrow_mut().take() {
+            Some(t) => t,
+            None => panic!("Format: was already formatted once"),
+        };
+
+        if let Some(fst) = iter.next() {
+            cb(&fst, f)?;
+            for elt in iter {
+                if self.sep.len() > 0 {
+                    f.write_str(self.sep)?;
+                }
+                cb(&elt, f)?;
+            }
+        }
+        Ok(())
+    }
+}
+
+macro_rules! impl_format {
+    ($($fmt_trait:ident)*) => {
+        $(
+            impl<'a, I> fmt::$fmt_trait for Format<'a, I>
+                where I: Iterator,
+                      I::Item: fmt::$fmt_trait,
+            {
+                fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+                    self.format(f, fmt::$fmt_trait::fmt)
+                }
+            }
+        )*
+    }
+}
+
+impl_format!{Display Debug
+             UpperExp LowerExp UpperHex LowerHex Octal Binary Pointer}