The subject of this thesis is the Mu-calculus, which nowadays represents a very active research area in both theoretical and practical Computer Science. The Mu-calculus is a logic capable of expressing least and greatest solutions of fixpoint equations x=f(x), where f is a monotone function mapping some complete ordered set into itself. The thesis has two goals, one expository and one technical. The expository goal is to present some results and problems about the Mu-calculus. On the technical respect, this thesis addresses a specific expressiveness problem of the Mu-calculus, the alternation depth hierarchy problem.