This chapter examines morphological processes in sign languages, with an eye toward understanding morphology that changes internal properties of a sign. Cross-linguistic comparisons of German, Japanese and American Sign Languages reveal two such types of morphological processes. One changes the sign according to fixed forms listed in the lexicon; the other looks to interaction with gestural space to determine its realization. While both are subject to language-specific constraints against marked forms, only the latter is also subject to phonological constraints against moving or twisting a manual articulator. These constraints arise because interaction with gestural space has the potential to result in forms that exceed the limits of the articulators. This latter type of nonconcatenative morphology makes sign languages unique.