This monograph describes an invariant, lambda, of oriented rational homology 3-spheres, which is a generalization of Andrew Casson's work in the integer homology sphere case. A formula describing how lambda transforms under Dehn surgery is provided. The formula involves Alexander polynomials and Dedekind sums, and can be used to give a rather elementary proof of the existence of lambda. It is also shown that when M is a Z2-homology sphere, lambda (M) determines the Rochlin invariant of M.