Our primary focus is on analysis of the concept of voluntariness, with a secondary focus on the implications of our analysis for the concept and the requirements of voluntary informed consent. We propose that two necessary and jointly sufficient conditions must be satisfied for an action to be voluntary: intentionality, and substantial freedom from controlling influences. We reject authenticity as a necessary condition of voluntary action, and we note that constraining situations may or may not undermine voluntariness, depending on the circumstances and the psychological capacities of agents. We compare and evaluate several accounts of voluntariness and argue that our view, unlike other treatments in bioethics, is not a value-laden theory. We also discuss the empirical assessment of individuals' perceptions of the degrees of noncontrol and self-control. We propose use of a particular Decision Making Control Instrument. Empirical research using this instrument can provide data that will help establish appropriate policies and procedures for obtaining voluntary consent to research.