Life-history strategies, ranging from completely free-living types to obligate entoparasitism, through ectocommensalism, ectoparasitism, and entocommensalism, are discussed within the framework provided by the concepts of r- and K-selection. In r-selection density and competition effects are minimal and progeny output (fecundity) can be maximised, while in K-selection competition is keen and the premium is on adult survival with production of a smaller number of extremely fit offspring. Thus r-strategists, with high fecundity, can be expected to have low calorific values because their resources are channelled into production of the maximum number of progeny, while K-strategists will have high calorific values based on lipid reserves which buffer the adults against possible reductions in food supply. This negative relationship between fecundity and calorific value is found in the phylum Platyhelminthes, with entosymbiotes at one end of the spectrum (high fecundity, low calorific value) and ectocommensals and free-living species at the other (low fecundity, high calorific value). This puts the entosymbiotes as more r-selected than the ectocommensal and free-living species, with the ectoparasites occupying an intermediate position. However, it is argued that parasites, and particularly entoparasites, follow both an r- and K-strategy at the same time and that this is only possible because of the stable, nutrient-rich environment provided by the host. Evolutionary theory dictates that all species would follow an r- and K-strategy simultaneously but environmental conditions force them into one alternative or another. Consequently the high fecundity of entoparasites, which has hitherto been viewed as a specific adaptation to entoparasitism, is now viewed as an automatic consequence of the conditions provided by the parasitic environment.