Abstract: Plant competition plays an important role in the success and sustainability of agroforestry systems. Minimizing competition and maximizing resource utilization increase crop yield and overall productivity of agroforestry systems. This paper introduces the concept and theory of plant competition, reviews competition for light, water and nutrient among trees and crops, and summarizes allelopathy and competition related plant traits in agroforestry systems. Aboveground competition for light among trees and crops is a major constraint under humid climate conditions, while belowground competition for water predominates in semi-arid tropics and temperate agroforestry systems. In agroforestry systems, plant competitive ability for light is decided by leaf area, canopy structure, et al. Root traits related to belowground competitive ability include occupied soil space, morphological and physical elasticity and spatial and temporal soil partitions. It is suggested that future research on agroforestry systems focus on comparisons among different components in different climatic conditions, competition processes and physiological mechanisms, plant allelopathy and the role and mechanism of mycorrhizae in plant competition, interactions between above- and below-ground competition and the effect of global climate change on plant competition of the agroforestry systems.