Susanne Schwinning and Jacob Weiner review the mechanisms that determine the degree of size asymmetry in plant competition. Size-asymmetric competition occurs when larger plants obtain a disproportionate share of resources, suppressing smaller neighbors. They distinguish between mechanisms based on resource characteristics and those influenced by plant growth and behavior. A resource must be "pre-emptable" to generate size-asymmetric competition, with light being a primary example. Competition for mineral nutrients is often size symmetric, but the role of patchily or episodically supplied nutrients in causing size asymmetry is underexplored. Water competition's size symmetry is poorly understood. Morphological and physiological plasticity can reduce size asymmetry. An allometric perspective on growth, resource uptake, and utilization helps understand competition mechanisms. The review clarifies terminology for competition modes, ranging from completely size symmetric to completely size asymmetric. Competition modes are often inferred indirectly from size-growth relationships in crowded populations, but alternative explanations exist. Quantifying asymmetric competition requires distinguishing between competitive and intrinsic effects. Evidence for size-asymmetric competition includes density effects on size variation and neighbor effects on individual growth. Patterns of size-symmetric and size-asymmetric competition vary. Competition is often partially size asymmetric, but exceptions exist. Competition is not size asymmetric when plants grow from seed briefly, on poor soils, at low densities, or compete only for below-ground resources. Cases involving taller but not wider growth or clonal plants also show no size asymmetry. While intensity and size asymmetry may seem independent, competition intensity often increases with density. The hypothesis is that below-ground resource competition occurs over greater distances than light competition.Susanne Schwinning and Jacob Weiner review the mechanisms that determine the degree of size asymmetry in plant competition. Size-asymmetric competition occurs when larger plants obtain a disproportionate share of resources, suppressing smaller neighbors. They distinguish between mechanisms based on resource characteristics and those influenced by plant growth and behavior. A resource must be "pre-emptable" to generate size-asymmetric competition, with light being a primary example. Competition for mineral nutrients is often size symmetric, but the role of patchily or episodically supplied nutrients in causing size asymmetry is underexplored. Water competition's size symmetry is poorly understood. Morphological and physiological plasticity can reduce size asymmetry. An allometric perspective on growth, resource uptake, and utilization helps understand competition mechanisms. The review clarifies terminology for competition modes, ranging from completely size symmetric to completely size asymmetric. Competition modes are often inferred indirectly from size-growth relationships in crowded populations, but alternative explanations exist. Quantifying asymmetric competition requires distinguishing between competitive and intrinsic effects. Evidence for size-asymmetric competition includes density effects on size variation and neighbor effects on individual growth. Patterns of size-symmetric and size-asymmetric competition vary. Competition is often partially size asymmetric, but exceptions exist. Competition is not size asymmetric when plants grow from seed briefly, on poor soils, at low densities, or compete only for below-ground resources. Cases involving taller but not wider growth or clonal plants also show no size asymmetry. While intensity and size asymmetry may seem independent, competition intensity often increases with density. The hypothesis is that below-ground resource competition occurs over greater distances than light competition.