Autophagy - the degradation of organelles and cytoplasmic material - occurs through dynamic rearrangements of cellular membrane structures. Following the induction of autophagy, newly formed autophagosomes transfer cytosolic materials to the lysosome or vacuole for degradation. The autophagosome is an organelle destined for degradation, suggesting that the membrane structure is formed de novo many times. The autophagosome is formed through the nucleation, assembly and elongation of membrane structures. The concerted action of several Apg/Aut/Cvt proteins around a characteristic subcellular structure (the preautophagosomal structure) is the key to understanding this novel type of membrane-formation process.