This lecture examines how plants perceive and respond to other plants, how plant-plant interactions affect the growth and productivity of individual plants and the broader effects on plant communities. Plant-plant interactions can be competitive, such as competition for light and nutrients, but facilitation also occurs in plant communities. This lecture addresses the emerging understanding of the signals and cues that underpin plant-plant interactions, as well as how plants respond to those cues. Applications of this knowledge include mitigating the effects of invasive plants, and improving crop plant productivity by harnessing the power of plants to work with each other, and limiting their tendencies to act against each other. Posted May 31, 2013. www.plantcell.org/cgi/doi/10.1105/tpc.113.tt0513