This publication brings together over 25 years of accumulated knowledge about the biology and ecology of weeds in cotton that growers can put to practical use in weed management. The weeds studied include three perennials (bermudagrass, johnsongrass, and yellow nutsedge) and four annuals (ivyleaf, morninglory, barnyardgrass, black nightshade, and Palmer amaranth). Although applicable to some extent to all cotton growing states, this knowledge was developed under the irrigated growing conditions of the West (California, Arizona, and New Mexico) and may be most useful in weed management there. The publication addresses four major areas of weed biology and control: (1) plant characteristics that contribute to weed success, (2) weed reintroduction, (3) the influence of weed competition and weed-free periods on yields of cotton and on weed reproduction, and (4) the use of herbicides. The authors present information emphasizing how extensively weeds can damage cotton and suggesting how much weed control cotton can provide for itself.