Suppose that we have a semisimple, connected, simply connected algebraic group G with corresponding Lie algebra g. There is a Hopf pairing between the universal enveloping algebra U (g) and the coordinate ring O(G). By introducing a parameter q, we can consider quantum deformations U q (g) and O q (G) respectively, between which there again exists a Hopf pairing. We show that the category of crystals associated with U q (g) is a monoidal category. We define subgroups of U q (g) to be right coideal subalgebras, and subgroups of O q (G) to be quotient left O q (G)-module coalgebras. Furthermore, we discuss a categorical approach to subgroups of quantum groups which we hope will provide us with a link to crystal basis theory.• We occasionally omit the structure maps for algebras, Hopf algebras etc. , and simply refer to them as A, H etc.• id X denotes the identity map on X.• We refer to (co)unital, (co)associative (co)algebras as '(co)algebras'.