This book, now in its third edition, began almost 25 years ago when Weed Ecology: Implications for Vegetation Management was published in 1984. That text concentrated on the need for farmers, foresters, rangeland managers, and the researchers who advised them to understand better the biology of weeds and the role people play in creating and maintaining weeds in agriculture and other production systems. We were assisted in that first effort by the writings of many early scientists, such as J. L. Harper, H. G. Baker, and E. J. Salisbury, who studied the biology of weeds as a class of vegetation. We continue to be grateful for their pioneering.