  Theories of classification distinguish classes with some good structure
theorem from those for which none is possible. Some classes (dense linear
orders, for instance) are non-classifiable in general, but are classifiable
when we consider only countable members. This paper explores such a notion for
classes of computable structures by working out several examples. One
motivation is to see whether some classes whose set of countable models is very
complex become classifiable when we consider only computable members.
  We follow recent work by Goncharov and Knight in using the degree of the
isomorphism problem for a class to distinguish classifiable classes from non-
classifiable. For some classes (undirected graphs, fields of fixed
characteristic, and real closed fields) we show that the isomorphism problem is
\Sigma^1_1 complete (the maximum possible), and for others it is of relatively
low complexity. For instance, for algebraically closed fields, archimedean real
closed fields, and vector spaces, we show that the isomorphism problem is
\Pi^0_3 complete.
