Plastids are the descendents of once free-living cyanobacterial endosymbionts and are generally believed to have evolved only once during eukaryotic evolution, in a common ancestor shared by red, green, and glaucophyte algae. On multiple occasions, subsequent to this landmark event, plastids have moved horizontally across the eukaryotic tree by “secondary” endosymbiosis, a process in which a primary plastid-bearing alga becomes a permanent fixture inside an unrelated heterotrophic eukaryote. This chapter provides an overview of the diversity of secondary plastid-containing organisms, with special emphasis on those harboring plastids of red algal ancestry. The evolutionary “footprint” of secondary endosymbionts on the nuclear genomes of their hosts is discussed.