Fungi are crucial for terrestrial ecosystems, yet the role of fungal diversity in ecosystem functions remains unclear. We synthesize fungal biodiversity and ecosystem function (BEF) relationships, focusing on plant biomass production, carbon storage, decomposition, and pathogen or parasite resistance. The observed BEF relationships for these ecosystem functions vary in strength and direction, complicating generalizations. Strong positive relationships are generally observed when multiple ecosystem functions are addressed simultaneously. Often, fungal community composition outperforms species richness in predicting ecosystem functions. For more comprehensive fungal BEF research, we recommend studying natural communities, considering the simultaneous functions of a broader array of fungal guilds across spatiotemporal scales, and integrating community assembly concepts into BEF research. For this, we propose a conceptual framework and testable hypotheses.