The species is considered the basic unit of biological classification and evolution, although the ontological status of this category has always been a matter of debate. The inherently biological differences between the different kingdoms—such as animals, bacteria, and fungi—prevent the use of a single, universally applicable concept of species that could fit the mechanisms responsible for the variability present in these kingdoms. This controversy has given rise to opposite approaches: monism, which looks for a single definition that might account for all species, and pluralism, which admits that different groups of organisms require somewhat different definitions. This chapter aims to discuss the problem of the definition of species in reference to the new mechanistic approach. I argue that it is difficult to come up with an all-encompassing definition of species, but this does not render the very concept of species meaningless. It does not result that the mechanistic approach makes it possible to achieve a solution to the species problem, either from a conceptual or from an explanatory point of view. Instead, I will argue that the species problem is more understandable within a dual ontic-epistemic approach based on the distinction between the realist and nominalist stylizations of species definitions.