This chapter summarizes current ideas about various aspects of seed bank ecology. Although most plant communities have at least some persistent seeds in the soil, the size of seed banks varies greatly from one habitat to another. Thompson and Silvertown present tables of data on seed density in soils from a wide range of studies. There are various possible causes of the irregular distribution of seeds in the soil. Initial distribution may be concentrated around parent plants because of poor dispersal. To exploit a newly created gap, it is clearly important for the seeds in the seed bank to be able to detect it. Seeds can be considered to be placed in a gap if they have been brought to the unshaded surface of the soil, or at a depth from which they could emerge. Seed banks play a vital role in dynamics of most plant communities.