The signal recognition particle (SRP) is a protein-RNA complex that associates with ribosomes to mediate co-translational targeting of membrane and secretory proteins to biological membranes. The universally conserved core of SRP consists of SRP RNA and the SRP54 protein, and plays the key role in signal-sequence recognition and binding to the SRP receptor. Critical for SRP function is communication between the two conserved SRP54 domains, the GTPase- and the M-domain, so that signal-sequence binding at the M domain directs receptor binding at the GTPase domain. The structural basis for signal-sequence binding by SRP and subsequent signaling is still poorly understood. By studying the structures of the SRP RNA in its free form as well as in complex with its different protein partners, we have made steady progress towards the elucidation of structural states of the SRP, using the archaeon Methanococcus jannaschii as model system. Together with other structures of SRP proteins and RNA-protein complexes, these structures provide new insights into the mechanisms of SRP-mediated protein targeting.