This monograph is a revised version of the author's Ph.D. thesis, submitted to the University of Li?ge, Belgium, with Pierre Wolper as thesis advisor. The general pattern of this work, is to turn logical and semantic ideas into exploitable algorithms. Thus, it perfectly fits the modern trend, viewing verification as a computer-aided activity, and as algorithmic as possible, not as a paper and pencil one, dealing exclusively with semantic and logical issues. Patrice Godefroid uses state-space exploration as the key technique, which, as such or elaborated into model checking, is attracting growing attention for the verification of concurrent systems. For most realistic examples, the methods presented provide a significant reduction of memory and time requirements for protocol verification.