Secondary endosymbiosis is the process that drives the spread of plastids (chloroplasts) from one eukaryote to another. The number of times that this has occurred and the kinds of cells involved are now becoming clear. Reconstructions of plastid history using molecular data suggest that secondary endosymbiosis is very rare and that perhaps as few as three endosymbioses have resulted in a large proportion of algal diversity. The significance of these events extends beyond photosynthesis, however, because non-photosynthetic organisms such as ciliates appear to have evolved from photosynthetic ancestors and could still harbor plastid-derived genes or relict plastids.