Experimental evidence for positive interactions, or facilitation, among plants has increased markedly during the last 10 years. Experiments documenting facilitation have been conducted in many diverse ecological systems, which suggests that positive interactions may be fundamental processes in plant communities. Here, I review the evidence for facilitation, the mechanisms by which facilitation operates, and the effects facilitation has on community structure. Facilitative mechanisms may act simultaneously with resource competition or allelopathy, and the overall effect of one species on another may be the product of multiple, complex interactions. Positive interactions may also determine community spatial patterns, permit coexistence, enhance diversity and productivity, and drive community dynamics. Once viewed as anecdotal and idiosyncratic, facilitation is now contributing to a more complete understanding of community structure and dynamics.