In prokaryotic and eukaryotic organisms, proteins are efficiently sorted to reach their final destinations in a whole range of subcellular compartments. Targeting is mediated by hydrophobic signal sequences or hydrophilic targeting sequences depending upon the compartment, these sequences being often processed. Proteins cannot be translocated through a membrane in a tightly folded stage, they must have a loose conformation, the so-called 'translocation competent state', which is usually kept through interactions with chaperones. In addition to these cytosolic receptor-like components, receptors are also present on the target membranes. Depending upon the organelles and organisms, two different energy sources have been identified, energy rich phosphate bonds (ATP and GTP) and a potential across the target membrane. Besides the signal peptides, various classes of signals have been identified to account for topologies of membrane proteins. Protein secretion in bacterial organisms has been extensively studied. Various classes of proteins use different strategies, some of these may also be used in eukaryotic cells.