This paper presents the idea that biological species are natural kinds. Natural kinds represent a sort of a privileged way of grouping entities into classes based on some objective properties. Different views give different insights into what those objective properties are, but all recognize that they must be objective in some sense. Views that vary in their strictness of requirements for natural kinds permit different groups to be natural kinds. Chemical elements are the paradigm examples of natural kinds that follow the strictest requirements, meaning that they have a certain property that is enough to explain their membership of the kind. The property in question is their atomic number. However, finding such a property for biological species is not an easy task. We do not know what the ‘atomic number’ for biological species is, if it even exists, but regardless, we may assume that it does exist. It seems intuitive that the solution for this should be found in genes, however, it is generally accepted by philosophers of biology that genes cannot be the property that explains biological species membership in the sense in which the atomic number can explain the membership of chemical elements. Nonetheless, attempts are still being made to undermine this consensus and show that genes are an essential property of biological species.