The biogenesis of organelles and the maintenance of cell functions in multi-compartmentalized plant cells require a specific protein delivery mechanism to ensure efficient and effective translocation of proteins to their respective destinations. Increasing numbers of studies demonstrate that some proteins are targeted simultaneously to more than one compartment by a range of mechanisms, involving composite targeting sequences and/or transcriptional and translational controls. Recent data indicate that the final destination of a protein might respond to changes in the environment; this underlines the complexity of cell engineering that is required to localize a protein.