  However, Boltzmann's ideas on the precise relationship between the thermodynamical properties of macroscopic bodies and their microscopic constitution, and the role of probability in this relationship are involved and differed quite remarkably in different periods of his life. Indeed, in his first paper in statistical physics of 1866, he claimed to obtain a completely general theorem from mechanics that would prove the second law. However, thirty years later he stated that the second law could never be proved by mechanical means alone, but depended essentially on probability theory. In his lifelong struggle with the problem he employed a varying arsenal of tools and assumptions. (To mention a few: the so-called Stoßzahlansatz, the ergodic hypothesis, ensembles, the combinatorial argument, the hypothesis of molecular disorder.) However, the exact role of these assumptions, and the results he obtained from them, also shifted in the course of time.  Particularly notorious are the role of the ergodic hypothesis and the status of the so-called H-theorem. Moreover, he used ‘probability’ in four different technical meanings.  It is, therefore, not easy to speak of a consistent, single “Boltzmannian approach” to statistical physics. It is the purpose of this essay to describe the evolution of a selection of these approaches and their conceptual problems.