The profinite completion of the fundamental group of a closed, orientable 3-manifold determines the Kneser-Milnor decomposition. If M is irreducible, then the profinite completion determines the Jaco-Shalen-Johannson decomposition of M .When trying to distinguish two compact 3-manifolds M, N, in practice the easiest method is often to compute some finite quotients of their fundamental groups, and notice that there is a finite group Q which is a quotient of π 1 M, say, but not of π 1 N. It would be very useful, both theoretically and in practice, to know that this method always works. The set of finite quotients of a group Γ is encoded by the profinite completionΓ (the inverse limit of the system of finite quotient groups), and so one is naturally led to the following question.Question 0.1. Let M be a compact, orientable 3-manifold. To what extent is π 1 M determined by its profinite completion?In particular, if M is determined among all compact, orientable 3-manifolds by π 1 M, then M is said to be profinitely rigid. If there are at most finitely many compact, orientable 3-manifolds N with π 1 M ≅ π 1 N, then M is said