Horizontal gene transfer (HGT) may not only create genome mosaicism, but also introduce evolutionary novelties to recipient organisms. HGT in plastid genomes, though relatively rare, still exists. HGT-derived genes are particularly common in unicellular photosynthetic eukaryotes and they also occur in multicellular plants. In particular, ancient HGT events occurring during the early evolution of primary photosynthetic eukaryotes were probably frequent. There is clear evidence that anciently acquired genes played an important role in the establishment of primary plastids and in the transition of plants from aquatic to terrestrial environments. Although algal genes have often been used to infer historical plastids in plastid-lacking eukaryotes, reliable approaches are needed to distinguish endosymbionts-derived genes from those independently acquired from preferential feeding or other activities.