Nasal substitution, which replaces a base-initial obstruent with the homorganic nasal under prefixation, is arguably the most prominent morphonological process seen in Austronesian languages, as it is an active part of the verbal morphology of most languages of the Philippines and western Indonesia, as well as Malagasy, Chamorro, and Palauan. Although a comparative overview of this process was provided by Otto Dempwolff in the 1930s, and has been elaborated in several later studies, the full range of cross-linguistic variation in the form of nasal substitution has not previously been fully appreciated. This paper seeks to document the rich variability of the process of nasal substitution across languages, and show that recent attempts to discover a motivation for nasal substitution within the framework of Optimality Theory are inadequate, and are likely to remain so in any currently conceivable version of the theory.