WPF 4.0 custom dictionary support is still pretty weak. You can create a custom dictionary as a text file with a .lex extension, and manually add words to it that the spell checker will use in addition to its own built-in dictionary. But there is no way for the user to add an unrecognized word to the dictionary, short of exiting the app, opening the .lex file in Notepad, and typing it in. In other words, there is no "Add to dictionary" item on the spell-check context menu, even if a custom dictionary is in use. Custom dictionaries are useful, I suppose, for technical fields with well-defined terminology, which can be set up in a .lex file in advance. But the WPF spell-checker is pretty weak as a general purpose spell-checker.