The green unicellular alga Chlamydomonas reinhardtii occupies a unique position among photosyn- thetic organisms. Although its photosynthetic func- tion is very similar to that of vascular plants, it combines the advantages of unicellular organisms, which include fast growth under controlled environ- mental conditions with highly sophisticated genetics of the nuclear, chloroplast, and mitochondrial compartments. The aim of this article is to provide an overview of this powerful algal model system and to show that C. reinhardtii is ideally suited for studying the biogenesis of the photosynthetic apparatus, its structure-function relationship, and its remarkable ability to adapt to changing environmental conditions such as light quality and quantity and nutrient limitation.