Autophagy is a major intracellular degradation/recycling system ubiquitous in eukaryotic cells. It contributes to the turnover of cellular components by delivering portions of the cytoplasm and organelles to lysosomes, where they are digested. Autophagy is mediated by membrane trafficking of unique double-membrane structures, the so-called autophagosomes, which are formed transiently. Moreover, autophagy is dramatically induced under starvation conditions to maintain an amino acid pool so that essential proteins may be synthesized. Recent studies have revealed insights into the molecular basis of membrane dynamics and the regulation of autophagy, which had remained cryptic for a long time. (C) 2003 Elsevier Inc. All rights reserved.