Submanifold Theory is a branch of differential geometry that studies the properties and classification of submanifolds—differentiable manifolds embedded or immersed in a larger ambient manifold—by characterizing their intrinsic and extrinsic geometric structures, such as induced metrics, second fundamental forms, and curvature relations, thereby distinguishing them within the broader category of geometric theories through their dependence on both the internal geometry of the submanifold and its interaction with the surrounding space.