A quaternion manifold (or quaternion Kaehlerian manifold [10]) is defined as a Riemannian manifold whose holonomy group is a subgroup of Sp(l). Sp(m)=Sp(l)xSp(m)/{±1}. The quaternion projective space QP, its noncompact dual and the quaternion number space Q are three important examples of quaternion manifolds. It is well-known that on a quaternion manifold M, there exists a 3-dimensional vector bundle E of tensors of type (1, 1) with local cross-section of almost Hermitian structures satisfying certain conditions (see § 2 for details). A submanifold N in a quaternion manifold M is called a quaternion (respectively, totally real) submanifold if each tangent space of N is carried into itself (respectively, the normal space) by each section in E. It is known that every quaternion submanifold in any quaternion manifold is always totally geodesic. So it is more interesting to study a more general class of submanifolds than quaternion submanifolds. The main purpose of this paper is to establish the general theory of quaternion Ci?-submanifolds in a quaternion manifold which generalizes the theory of quaternion submanifolds and the theory of totally real submanifolds. It is proved in section 3 that such submanifolds are characterized by a simple equation in terms of the curvature tensor of a quaternion-space-form. In section 4 we shall study the integrability of the two natural distributions on a quaternion Ci?-submanifold. In section 5 we obtain some basic lemmas for quaternion Ci?-submamfolds. In particular, we shall obtain two fundamental lemmas which play important role in this theory. Several applications of the fundamental lemmas are given in section 6. In section 7 we study quaternion C7?-submanifolds which are foliated by totally geodesic, totally real submanifolds. In the last section we give an example of a quaternion Ci?-submanifold of an almost quaternion metric manifold on which the totally real distribution is not integrable.