This chapter describes what happens at the structural level to polysynthetic languages during language obsolescence, attrition, and loss. The changes that take place in decaying polysynthetic languages should be distinguished from (a) those occurring in all obsolescent languages regardless of their type, and (b) changes in “healthy” polysynthetic languages. It is shown that the consequences of polysynthetic language decay are primarily manifested in the collapse of morphological complexity, involving the loss of morphological ‘slots’, the reduction in the number of bound morphemes and their substitution by free ones, the ‘fossilization’ of markers and their reanalysis, the deprivation of word formation productivity, the destruction of noun incorporation, and reduction of allomorphy.