The emergence of functional ecology has changed the focus of ecological research from investigating patterns of species diversity to understanding how species traits relate to specific ecological processes generating these patterns. Traits, ecological patterns and processes can be shared and driven by species from distantly related taxonomic groups. Crossing the boundaries among distantly related taxonomic groups is still a challenge and a critical knowledge frontier in functional ecology. A cross-taxa approach, merging trait data across distantly related taxonomic groups, could fill this gap. In this context, functionally analogous traits, that is traits that may have distinct ontogenetic origins yet represent similar processes, comprise an important recent advance in functional ecology. However, which taxa and traits (be them analogous or not) have been used in research with multiple taxa, and whether (and how) these data have been combined, still needs to be elucidated. We reviewed articles published in the last 75 years to investigate the use of traits in functional research involving multiple taxa. Our search returned 1006 articles, and a subset of 96 was filtered for data extraction. Studies covered a total of 134 taxa and 491 different traits; they were predominantly observational, and focussed on community ecology and ecosystem monitoring. Our review showed that current knowledge in this field relies on a limited number of response variables, particularly taxonomic diversity (e.g. species richness and abundance within functional groups). Also, the field relies on a limited number of taxa (e.g. plants, birds and mammals) and trait types (diet, size, habitat and dispersal). Two-thirds of the articles (n = 72) used functionally analogous traits, and one-third of them (n = 32) employed a cross-taxa approach. We mapped the limitations of current research in functional ecology involving multiple taxa, presented ecological questions to a functional cross-taxa research and showed directions to pushing the limits of this research field. Our review aimed to encourage researchers in the field of functional ecology to move beyond single taxa and traits, and to integrate more branches and dimensions of the Tree of Life in their research. Read the free Plain Language Summary for this article on the Journal blog.