Given two structures M and N on the same domain, we say that N is a reduct of M if all emptyset-definable relations of N are emptyset-definable in M. In this thesis, the reducts of the generic digraph, the Henson digraphs, the countable vector space over F_2 and of the linear order Q.2 are classified up to first-order interdefinability. These structures are aleph_zero-categorical, so classifying their reducts is equivalent to classifying the closed groups that lie in between the structures’ automorphism groups and the full symmetric group.