Photosynthetic organisms have played a significant role as primary producers and ultimate sources of the atmospheric oxygen through geologic time. How they have evolved is one of the key questions in the study of the biological and environmental history of the Earth. Modern algal chloroplasts (plastids) are greatly diversified in morphology and pigmentation. Because the paleontological record can never be complete, a multidisciplinary approach is essential to reveal the pattern, timing, and mechanism of chloroplast evolution. Several independent lines of evidence show that all chloroplasts derived from a single endosymbiotic cyanobacterium and spread in different eukaryotic taxa via multiple secondary endosymbioses. With the advent of the genomic era, comparative genomics has been employed to reveal the course of their evolution. Yet, the study of non-model organisms remains important to understand how today's diverse life has evolved, such as in the case of uniquely pigmented prochlorophytes. Paleontological records may provide constraints on the timing of the primary and secondary endosymbiotic events.