Abstract We investigate the fundamental concept of a closed quantum subgroup of a locally compact quantum group. Two definitions — one due to S. Vaes and one due to S.L. Woronowicz — are analyzed and relations between them discussed. Among many reformulations we prove that the former definition can be phrased in terms of quasi-equivalence of representations of quantum groups while the latter can be related to an old definition of Podleś from the theory of compact quantum groups. The cases of classical groups, duals of classical groups, compact and discrete quantum groups are singled out and equivalence of the two definitions is proved in the relevant context. A deep relationship with the quantum group generalization of the Herz restriction theorem from classical harmonic analysis is also established, in particular, in the course of our analysis we give a new proof of the Herz restriction theorem.