Strongly correlated electron systems show a rich variety of interesting physical phenomena. However, their theoretical description poses a highly non-trivial task, and even the simplest models cannot be solved exactly. In this book the intricate behavior of the single impurity Anderson model and the Hubbard model is investigated. The main emphasis is laid on two-particle Green functions of these systems. To this end, existing approximation schemes are extended to be capable of treating susceptibilities as well. The collective magnetic and charge excitations of the above mentioned models are studied in detail. Magnetic phase transitions of the Hubbard model for various cubic lattices are described and interesting transition scenarios encountered.