I ntermediate filaments (IFs) are encoded by the largest gene family among the three major cytoskeletal protein groups. Unique IF compliments are expressed in selective cell types, and this expression is reflected in their involvement, upon mutation, as a cause of or predisposition to more than 80 human tissue-specific diseases. This Review Series covers diseases and functional and structural aspects pertaining to IFs and highlights the molecular and functional consequences of IF-associated diseases (IF-pathies). Exciting challenges and opportunities face the IF field, including developing both a better understanding of the pathogenesis of IF-pathies and targeted therapeutic approaches.