Recent attempts to understand the development of intense precipitation has led several investigators to speculate about the role of drop impactions. Working with a large vertical wind tunnel, investigations were carried out on the interactions of millimeter size drops. On the basis of these studies it is concluded that drop impactions produce a rapid increase in the number of precipitation size drops (average four or five per collision) while limiting the growth of larger drops. For drops ≳4 mm in diameter there exists a spectrum of smaller drops whose relative kinetic energy of impaction exceeds the critical value of 15 ergs, thus preventing permanent coalescence. However, impactions seldom completely destroy the larger drops, but they do remove some mass. Drop impactions reveal a self-regulating mechanism in nature that enables collisions to influence both the initial growth and the determination of final size for large drops.