The aim of this chapter is to ask what aspects pertaining to the evolutionary origin of multicellular life might be inferred from a survey of otherwise dissimilar protists that display one or both of two features: a unicellular-to-multicellular transition as part of their normal life cycle, or membership of a closely related group that contains both unicellular and multicellular members. Accordingly, we highlight aspects of multicellular development in three different "supergroups" of eukaryote protists: Dictyostelid or cellular slime molds (CSMs) (supergroup Amoebozoa, Mycetozoa; Section 4.2); Choanoflagellates, Filastareans, and Icthyosporeans (Opisthokonta, unicellular Holozoa; Section 4.3); and Volvocine green algae (Archaeplastida, Chlorophyceae; Section 4.4); their last common ancestor is believed to lie at the very root of the evolutionary tree of eukaryotes (Burki). In the first two, multicellularity is achieved by the aggregation of single cells, in the third, by the products of cell division staying together.