The formulation of quantum mechanics in rigged Hilbert spaces is used
to study the vector states for resonance states or Gamow vectors. An i
mportant part of the work is devoted to the construction of Gamow vect
ors for resonances that appear as multiple poles on the analytic conti
nuation of the S-matrix, S(E). The kinematical behavior of these vecto
rs is also studied. This construction allow for generalized spectral d
ecompositions of the Hamiltonian and the evolutionary semigroups, vali
d on certain locally convex spaces. Also a first attempt is made to de
fine the resonance states as densities in an extension of the Liouvill
e space, here called rigged Liouville space.