Sex determination processes are those developmental processes that lead to the specific sexual characteristics of an individual. As biologists we can ask two major types of questions about these processes: (i) How do sexual characteristics develop, how variable are they, and what conditions affect the developmental trajectories taken? (ii) How do these developmental processes themselves evolve? From an epigenetic (developmental) perspective the two questions – the developmental-epigenetic question and the evolutionary question – are closely related. The analysis of evolutionary change starts from an understanding of the developmental processes that lead to condition-dependent phenotypic variability. Only when we understand how environmental factors (e.g. uterine, ecological, psychological, social) influence phenotypic variability, can we suggest how evolutionary change had occurred. Not only is the developmental perspective important for the identification of the regulatory genetic factors that are the likely foci of evolutionary change, but the understanding of developmental flexibility also allows the construction of likely evolutionary scenarios, with the epigenetic environmentally-induced developmental changes often initiating and guiding the evolutionary process. What exactly is epigenetics and what are epigenetic changes? I define Epigenetics as the study of the processes that bring about persistent condition-dependent developmental effects. At the cellular level, these are the processes involved in cell determination and differentiation. At higher levels, epigenetic mechanisms generate the context-dependent, self-sustaining interactions between groups of cells that lead to physiological, behavioral and morphological persistence. Epigenetic inheritance is a component of epigenetics: it occurs when phenotypic variations (e.g. variations in gene expression) that do not stem from variations in DNA base sequences are transmitted to subsequent generations of cells or organisms. I will describe some epigenetic mechanisms and will give examples that demonstrate their relevance for understanding the development of sexual characteristics in rodents. I will end with some speculations about the evolution of the unusual sexual characteristics Spotted Hyena females, and about the evolution of chromosomal sex determination in mammals.