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+# Functional Language Features: Iterators and Closures
+
+Rust’s design has taken inspiration from many existing languages and
+techniques, and one significant influence is *functional programming*.
+Programming in a functional style often includes using functions as values by
+passing them in arguments, returning them from other functions, assigning them
+to variables for later execution, and so forth.
+
+In this chapter, we won’t debate the issue of what functional programming is or
+isn’t but will instead discuss some features of Rust that are similar to
+features in many languages often referred to as functional.
+
+More specifically, we’ll cover:
+
+* *Closures*, a function-like construct you can store in a variable
+* *Iterators*, a way of processing a series of elements
+* How to use closures and iterators to improve the I/O project in Chapter 12
+* The performance of closures and iterators (Spoiler alert: they’re faster than
+  you might think!)
+
+We’ve already covered some other Rust features, such as pattern matching and
+enums, that are also influenced by the functional style. Because mastering
+closures and iterators is an important part of writing idiomatic, fast Rust
+code, we’ll devote this entire chapter to them.