This Special Issue on Morphological Processing is based on the sixth MOrphological PROcessing Conference (MOPROC), which was kept in June 2009 in Turku, Finland. The issue contains 13 articles by leading scholars in the field of morphological processing. These articles investigate the role morphemes play in language comprehension, production and acquisition. Specific questions relate to the time course with which morphemes come available, what factors facilitate their use, the role of orthographic and semantic transparency in complex word processing and how morphology should be incorporated in models of word processing. The papers in this issue provide a wealth of empirical results in several languages obtained with state-of-the-art experimental paradigms and will be an inspiration for further studies in morphological processing. (Contains 1 table.)