This volume summarizes over 20 years of research activity by the scientific community into the phenomenon of targeting proteins, the study of how newly synthesized proteins reach their ultimate destination. This may be an intracellular organelle such as the nucleus, mitochondria or rough endoplasmic reticulum, or it may be secreted or taken up by a cell and distributed to specific locations such as the endosome or lysosome. Small parts of the protein's structure are recognized by intracellular receptors, which direct or "target" the protein to its destination. Some proteins must cross membranes during this process, and many are subject to modification as they travel. The understanding of protein targeting has many applications, including cancer research, the study of diseases resulting from faulty targeting and the genetic manipulation of micro-organisms to target useful proteins.