This monograph is the first effort by the American Society of Agronomy, Crop Science Society of America, and the Soil Science Society of America to publish a book on the use of computer simulation to evaluate agronomic systems. It is a testament to the desire of agronomists to enter a new era of agricultural research and development; a time in which simulation partially substitutes for experiments to determine recommendations for various agrotechnology packages. Improved accuracy in simulating crop and soil systems has convinced a growing number of researchers and technologists of the importance of being able to predict outcomes needed in agricultural decision making. Combining the soil, plant, and climate system into quantitative terms that lead to accurate predictions of outcomes is needed as agronomists enter the information age.