I discuss the continuum of approaches that exist when studying sexual or natural selection in the wild and the laboratory. These range from behavioural observations in the laboratory, via experimental manipulations of particular traits or environments, to phenomenological studies in nature. I focus on the study of body size and related life history traits, particularly drawing from our own studies on dung flies as examples. For any given species or phenomenon, ideally all types of studies should be integrated to obtain a complete picture of the evolution of particular traits in terms of mechanisms and function, proximate and ultimate explanations. I particularly advocate the use of standardised selection measures, which are well established in the literature but underused, and which I discuss in the chapter as practical guidance for the general reader. Utilisation of such measures even in experimental, laboratory studies of behavioural mechanisms, which is often possible but rarely done, would greatly facilitate any future meta-analyses of particular traits, species and evolutionary phenomena.