The role of circumstellar disks in star and planetary formation is briefly reviewed. The observed disk around MWC 349 is used as an example and a table of evolutionary time scales and parameters is presented. The disk about MWC 349 is characteristic of that expected about a massive star. Disk structure about solar mass stars is more completely reviewed by Cameron (1978). The parameters for the disk indicate that there is a deduced region where conditions are appropriate for dust condensation and possible aggregation of material to planetary masses. For the purposes of the discussion we are assuming that the infrared as well as optical radiation arises from the disk which extends the ‘known’ extent of the disk to 1014cm. It is not yet certain that this is the case.