Highly collimated jets are associated with young stellar objects of all masses. They are also prevalent in microquasars and a variety of active galactic nuclei. Common to all of these systems is the presence of accretion discs around central objects, be they young stars, compact objects, or black holes. Current observations of protostellar systems support a model that posits accretion–powered, centrifugally accelerated, hydromagnetic disc winds as the underlying mechanism for protostellar jets. This model also has the attractive possibility of unifying protostellar, microquasar and extragalactic jets within a single conceptual framework. We summarize the observations, theory and time–dependent numerical simulations of disc winds as the origin of protostellar jets. We also address the relevance of these winds for the theory of star formation, as well as the more general issue of a unified theory for astrophysical jets.