We consider the question of when a semigroup is the semigroup of a valuation dominating a two-dimensional noetherian domain, giving some surprising examples. We give a necessary and sufficient condition for the pair of a semigroup S and a field extension L/k to be the semigroup and residue field of a valuation dominating a regular local ring R of dimension 2 with residue field k, generalizing the theorem of Spivakovsky for the case when there is no residue field extension.