This chapter will describe work aiming to determine the origins of the autophagosomal membrane, which is a topic as old as the study of autophagy itself. Several potential ideas have been proposed over the years with many cellular membranes being considered as sites of autophagosome formation, and including the possibility that autophagosomes form de novo in the cytoplasm without interaction with any other membrane. A consensus view that has emerged and will be discussed is that autophagosomes form in primary association with the endoplasmic reticulum (ER) but also in secondary contact with other membrane compartments. Contact with the ER allows transfer of lipids to the forming autophagosomal membrane, and it also provides a cradle that shapes the formation of autophagosomes. To arrive at this conclusion, a large number of laboratories used morphological, biochemical and structural studies to define and describe each step of the process and to understand the protein machineries involved in functional and structural detail. These approaches will be described in this chapter as a way to introduce you to a variety of experimental work used in the study of many cell biological problems, including autophagy. Finally, we will dedicate a brief section to the comparison of autophagosome formation mechanisms in selective and non-selective autophagy.