  Landstad-Vaes theory deals with the structure of the crossed product of a
C$^*$-algebra by an action of locally compact (quantum) group. In particular it
describes the position of original algebra inside crossed product. The problem
was solved in 1979 by Landstad for locally compact groups and in 2005 by Vaes
for regular locally compact quantum groups. To extend the result to non-regular
groups we modify the notion of $G$-dynamical system introducing the concept of
weak action of quantum groups on C$^*$-algebras. It is still possible to define
crossed product (by weak action) and characterise the position of original
algebra inside the crossed product. The crossed product is unique up to an
isomorphism. At the end we discuss a few applications.
