  Streams, or infinite sequences, are infinite objects of a very simple type,
yet they have a rich theory partly due to their ubiquity in mathematics and
computer science. Stream differential equations are a coinductive method for
specifying streams and stream operations, and their theory has been developed
in many papers over the past two decades. In this paper we present a survey of
the many results in this area. Our focus is on the classification of different
formats of stream differential equations, their solution methods, and the
classes of streams they can define. Moreover, we describe in detail the
connection between the so-called syntactic solution method and abstract GSOS.
