The biogenesis of autophagosomes entails the nucleation and growth of a double-membrane sheet, the phagophore, which engulfs cytosol for delivery to the lysosome. Genetic studies have identified a class of Atg proteins that are essential for the process, yet the molecular mechanism of autophagosome biogenesis has been elusive. Proteomic, structural, super-resolution imaging, and biochemical reconstitution experiments have begun to fill in some of the gaps. This review describes progress and prospects for obtaining a four-dimensional network model of the nucleation and growth of the phagophore. © 2014.