DescriptionWe study the fundamental groups of open n-manifolds of non-negative Ricci curvature, via the method of Gromov-Hausdorff convergence. In 1968, Milnor conjectured that any open n-manifold M of non-negative Ricci curvature has a finitely generated fundamental group. In this thesis, we verify this conjecture under various geometrical conditions. We show that the Milnor conjecture holds when M has dimension 3, or when the Riemannian universal cover of M has Euclidean volume growth and the unique tangent cone at infinity, or when pi_1(M)-action on the Riemannian universal cover satisfies the no small almost subgroup condition.