Gene flow in seed plants refers to the movement of genes within and between plant populations. This has long been of interest to plant breeders and seed producers, with economic factors driving studies on gene flow based on distance, breeding systems, pollination agents, and planting designs. In recent decades, plant evolutionists have become increasingly interested in gene flow in wild populations, focusing on topics such as pollen and seed dispersal mechanisms, plant-pollinator coevolution, and species boundaries. Early evidence suggested that gene flow was extensive, but this view has been challenged as more data became available and was critically analyzed. There is considerable evidence that gene exchange between populations may be much lower than previously thought. Information on gene flow in plants is scattered across various scientific literature. Wolfenbarger compiled extensive reviews of the literature before 1960, and this chapter extends that survey and develops the Ehrlich and Raven (1969) thesis on gene flow restriction. It aims to organize information on pollinator foraging, pollen flow, seed dispersal, and gene flow within and between populations. It also seeks to explain the implications of observed gene flow levels for the breeding structure of plant populations and systems. "Potential gene flow" refers to the deposition of pollen and seeds from a source as a function of distance, while "actual gene flow" refers to the incidence of fertilization and establishment of reproductive individuals as a function of distance. There may be a significant difference between actual and potential gene flow. However, the movement of pollen and seeds provides valuable insights into actual gene dispersal without genetic markers or knowledge of gene frequencies. Most species disperse pollen through animals or wind. Animal pollination is more efficient, increasing seed set and reducing the number of pollen grains needed per ovule. Animal pollination is common in closed communities, especially tropical rainforests. Wind pollination is more common in open, often arid regions, such as prairies, temperate forests, and savannas.Gene flow in seed plants refers to the movement of genes within and between plant populations. This has long been of interest to plant breeders and seed producers, with economic factors driving studies on gene flow based on distance, breeding systems, pollination agents, and planting designs. In recent decades, plant evolutionists have become increasingly interested in gene flow in wild populations, focusing on topics such as pollen and seed dispersal mechanisms, plant-pollinator coevolution, and species boundaries. Early evidence suggested that gene flow was extensive, but this view has been challenged as more data became available and was critically analyzed. There is considerable evidence that gene exchange between populations may be much lower than previously thought. Information on gene flow in plants is scattered across various scientific literature. Wolfenbarger compiled extensive reviews of the literature before 1960, and this chapter extends that survey and develops the Ehrlich and Raven (1969) thesis on gene flow restriction. It aims to organize information on pollinator foraging, pollen flow, seed dispersal, and gene flow within and between populations. It also seeks to explain the implications of observed gene flow levels for the breeding structure of plant populations and systems. "Potential gene flow" refers to the deposition of pollen and seeds from a source as a function of distance, while "actual gene flow" refers to the incidence of fertilization and establishment of reproductive individuals as a function of distance. There may be a significant difference between actual and potential gene flow. However, the movement of pollen and seeds provides valuable insights into actual gene dispersal without genetic markers or knowledge of gene frequencies. Most species disperse pollen through animals or wind. Animal pollination is more efficient, increasing seed set and reducing the number of pollen grains needed per ovule. Animal pollination is common in closed communities, especially tropical rainforests. Wind pollination is more common in open, often arid regions, such as prairies, temperate forests, and savannas.