The chapter reviews the mechanisms that determine the degree of size asymmetry in competition among plants. It distinguishes between mechanisms influenced by the nature of contested resources and those influenced by the growth and behavior of the plants themselves. Light is highlighted as a primary example of a pre-emptable resource, leading to size-asymmetric competition. In contrast, competition for mineral nutrients is often size symmetric, while the role of patchily or episodically supplied nutrients in causing size asymmetry is underexplored. The chapter also discusses the plasticity in morphology and physiology that reduces the degree of size asymmetry. It emphasizes the importance of an allometric perspective on growth, allocation, resource uptake, and utilization to understand and quantify the mechanisms of plant competition. The introduction highlights the need to bridge the gap between physiological mechanisms of resource uptake and the performance of individuals and populations mediated by competition. The chapter further explores the patterns of size-symmetric and size-asymmetric competition, noting that competition is usually partially size asymmetric but can vary depending on factors such as plant density, soil quality, and resource type.The chapter reviews the mechanisms that determine the degree of size asymmetry in competition among plants. It distinguishes between mechanisms influenced by the nature of contested resources and those influenced by the growth and behavior of the plants themselves. Light is highlighted as a primary example of a pre-emptable resource, leading to size-asymmetric competition. In contrast, competition for mineral nutrients is often size symmetric, while the role of patchily or episodically supplied nutrients in causing size asymmetry is underexplored. The chapter also discusses the plasticity in morphology and physiology that reduces the degree of size asymmetry. It emphasizes the importance of an allometric perspective on growth, allocation, resource uptake, and utilization to understand and quantify the mechanisms of plant competition. The introduction highlights the need to bridge the gap between physiological mechanisms of resource uptake and the performance of individuals and populations mediated by competition. The chapter further explores the patterns of size-symmetric and size-asymmetric competition, noting that competition is usually partially size asymmetric but can vary depending on factors such as plant density, soil quality, and resource type.