It is well-known that any compact Lie group appears as closed subgroup of a unitary group, G ⊂ U N . The unitary group U N has a free analogue U + N , and the study of the closed quantum subgroups G ⊂ U + N is a problem of general interest. We review here the basic tools for dealing with such quantum groups, with all the needed preliminaries included, and we discuss as well a number of more advanced topics.2010 Mathematics Subject Classification. 46L65.