Acyclic orientations with exactly one source and one sink - the so-called bipolar orientations - arise in many graph algorithms and specially in graph drawing. The fundamental properties of these orientations are explored in terms of circuits, cocircuits and also in terms of ``angles'' in the planar case. Classical results get here new simple proofs; new results concern the extension of partial orientations, exhaustive enumerations, the existence of deletable and contractable edges, and continuous transitions between bipolar orientations.