When you migrate from a free Smalltalk, such as Smalltalk Express, to a commercial Smalltalk, such as VisualWorks(TM), you find that the core of the language changes little, but that there are important extensions that increase your productivity. Category and protocol grouping and input–output extensions are described in this chapter. Other extensions, described in subsequent chapters, deal with the extremely important model–viewer–controller paradigm and the construction of GUIs.