Chloroplasts contain approximately 3500 proteins, but only a small fraction of these proteins (approximately 100) are encoded in the chloroplast genome. Most chloroplast proteins are encoded in the nucleus and have to be imported post-translationally across the translocon of the inner and outer envelope into the organelle. For the translocation of proteins from the stroma to the thylakoid membrane or across the membrane into the thylakoid lumen four different pathways have been described so far: the Sec-pathway and the Tat-pathway are mainly involved in the transport of lumenal proteins, whereas the spontaneous-pathway and the cpSRP-pathway are used for the insertion of integral membrane proteins into the thylakoid membrane. Current research projects of our group are the detailled structure-function analysis of proteins that are known to be involved in the targeting of proteins to the thylakoid membrane, identification of novel components required for protein transport to or across the thylakoid membrane and analysis of the regulation of the cpSRP-dependent protein transport.