Milling clusters, surges within gatherings, street actions, demonstration marches, and state processions are forms of social behavior with which we mark points along a continuum of collective locomotion. A theoretical framework, field observations, measurement criteria and procedures are presented to systematically describe variations in the complexity of collective locomotion. An explanation for these variations is drawn from the ideas of G. H. Mead and from the cybernetic model of W. T. Powers. Field observations and quasi-experimental evidence are presented in support of that explanation. Implications are discussed for the recharacterization and explanation of, and for future research on, other elementary forms of collective behavior.