During the past few years, 2D (two-dimensional) materials have found increasing attention in the electronic device community. The first 2D material studied in detail was graphene and many groups explored it as a material for transistors. During the early years of graphene research, the expectations on its impact on electronics have been extremely high. It soon turned out, however, that the missing bandgap of graphene causes problems for proper transistor operation and meanwhile the prospects of graphene are assessed less optimistic. Recently researchers have extended their work to 2D materials beyond graphene and the number of 2D materials under investigation is literally exploding. At present, about 500 2D materials are known and part of them is considered to be useful for electronic applications. A realistic assessment of the prospects of the 2D materials, however, is still missing. The present paper represents is a step in this direction. After introducing the major classes of 2D materials, we compose a short wish list of material properties desirable for transistor channels and examine to what extent the 2D materials fulfill the criteria of our wish list. We review the current state-of-the-art of 2D transistors, compare their performance to that of competing conventional transistors, and identify potential applications of 2D materials and transistors.