We know that many species contain chloroplasts. In most cases, these species descend directly from a common ancestor that acquired the chloroplast through an endosymbiotic relationship with a cyanobacterium. The endosymbiotic origin of chloroplasts from cyanobacteria is not in doubt.
What is in doubt is whether the original endosymbiosis happened just once or whether there are multiple independent origins of chloroplasts. We also know that some species acquired chloroplasts by fusing with another chloroplast-containing eukaryotic species. How many examples of secondary acquisition are required to explain the phylogeny of species that contain chloroplasts? Are there tertiary and quaternary acquisitions?
Christopher Taylor of Catalogue of Organisms has posted a nice summary of the problems in this field at Crossing the Algal Divide. If you want to keep up with one of the important problems in evolution then this is an excellent place to start.
[Photo Credit: micro*scope]