Eliot Miranda has written an excellent series of articles on the implementation of Block Closures that Squeak, and virtually all of its derivatives such as Pharo and Cuis, are now implementing.

In many other languages and environments (including commercial Smalltalk implementations), closures are old news and have been around for years. Squeak is a little different since it was originally based on the Smalltalk-80 code which did not implement blocks as full closures (but that doesn't mean it hasn't been an issue for a long time.) So while this isn't ground-breaking in the world of languages, it is an important feature as it catches Squeak up in an area where it had been lacking.

Part I discusses the problem and the solution he found, part II covers implementing VM support, and part III wraps up with adding support to the compiler.