Publisher Summary This chapter discusses the contact inhibition of locomotion. Contact inhibition is defined as the stopping of the continued locomotion of a cell in the direction which has produced a collision with another cell or—alternatively, the prohibition when contact between cells has occurred—of continued movement such as would carry one cell across the surface of another. The original data on contact inhibition of locomotion were obtained from studies on the displacement of the whole cell. Explants were placed so that the cellular outgrowths met between them. The formation of a more-or-less single layer of cells over the substrate and the virtual absence of nuclear overlaps rapidly came to be used as criteria for the presence of contact inhibition, while a high incidence of nuclear overlap, multilayering, or piling up, was considered evidence of its absence. The amount of overlap in a particular situation was expressed as an overlap index. This value was derived by estimating the number of cell nuclei which would be expected to overlap if the cells were distributed at random and comparing this number with the number of overlaps actually occurring, the ratio of the observed number to the expected number being the index.