Basic concepts regarding forces and moments applied during orthodontic treatment returned to prominence in clinical orthodontics in the early 1960s and subsequently have been reviewed again and again in textbooks, journal articles, and presentations. This classic paper is one of the first to explain the attributes of forces (magnitude, continuity, constancy, direction), criteria for force evaluation (pain, rate of movement, tooth mobility, anchorage conservation, histopathology), and utilization of engineering terms such as moments. World J Orthod 2003;4:253–257.