Species are different, but they are not equally different. Yet many indices of biodiversity assume species vary to identical degrees. This notion does not meet with intuition: some species vary greatly in terms of their morphology, behaviour and ecology, while others vary only a little. One way to reconcile the dissimilarity between species is by collecting information on their functional traits (FTs), descriptors of how organisms interact with their environment and each other. Functional diversity (FD) is the total variation in one or more FTs across all species within a community, and provides a powerful complement to species diversity. There are several challenges facing the application of FD to conservation science, including lack of rigorous trait data for many organisms, and sparse details on how to select available traits to generate meaningful inferences for the various summary metrics of FD. This Comment provides a brief discussion on choosing and using FTs, and recommendations for best practice. Ultimately, researchers need to consider using a variety of traits when hypotheses are multifaceted or could potentially evolve, at the same time thinking critically about trait selection to avoid redundant information.