A genetic analysis of Chlamydomonas reinhardtii has revealed a large number of genes that are required for the expression of the chloroplast genome. These genes are surprisingly numerous and most of them are located in the nucleus. They are often specific for individual chloroplast genes and are required in post-transcriptional steps of gene expression. A demonstrative example is provided by the psaA gene, which is expressed from scattered exons by a mechanism that calls for two steps of RNA splicing in trans. This complex process requires the activity of more than 14 nuclear genes as well as one chloroplast locus. Our current understanding of this case of tight coupling between the nuclear and chloroplast genomes is reviewed here.