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Chloroplasts are highly structured plant specific organelles. They possess three discrete membrane systems that differ in composition and function, i.e. the outer/inner envelope and the thylakoid membranes. In addition three solute spaces can be distinguished, i.e. the space between the envelope membranes, the stroma and the thylakoid lumen. While most of the chloroplastic proteins, which are synthesized as precursors in the cytosol, seem to follow a common route of translocation into the organelle, proteins of the outer envelope, which is in direct contact with the cytosol, are inserted (imported) by a very different and distinct mechanism. A typical polypeptide destined for the inside of the organelle, possesses a cleavable target sequence, retains a loosely folded conformation with the help of molecular chaperones, is recognized by proteinaceous receptors on the organellar surface, requires low concentration (pM) ATP for binding but high concentrations (mM) for complete translocation through the membranes. Outer envelope polypeptides (OEP) studied so far, do not possess a cleavable target sequence, do not require protease sensitive receptors on the organellar surface and do not require ATP for either binding or insertion into the outer envelope.