Multiple pathways of intracellular protein breakdown operate within cells, and the activities of different pathways can be regulated under different physiological conditions. Recent studies suggest that molecular determinants within proteins target them for different pathways of proteolysis. Proteins that are partially unfolded and have an unblocked amino-terminal amino acid with a bulky side chain appear to be good substrates for cytosolic, ubiquitin-mediated pathways of proteolysis. Certain modifications of internal residues such as oxidation of methionines also increase the susceptibility of certain proteins to ubiquitin-mediated proteolysis. Rapidly degraded normal proteins contain peptide regions rich in proline, glutamate, serine, and threonine (PEST regions). The pathway of degradation for these proteins has not been established, but they may be good substrates for calcium-activated proteases. In addition, a lysosomal pathway of protein degradation is activated when serum is withdrawn from cultured cells and is selective for cytosolic proteins containing peptide regions similar to Lys-Phe-Glu-Arg-Gln (KFERQ). This short review summarizes our current understanding of mechanisms of protein breakdown in eukaryotes and evaluates potential molecular determinants of protein half-lives.