Haptic feedback is a design element for human-computer interfaces, and this paper discusses when and how it can be used to best effect in interactive applications. It begins with consideration of the unique attributes of the touch sense in physiological and psychological terms, and the nature of information and control that touching provides. It reviews where active touching helps, by setting forth the forms it may take and important parameters that describe it; and evaluates the specific benefits it offers to contemporary interface problems. It ends with a proposal for a simple interaction model that emphasizes holistic design principles, and highlights issues that arise in the process of creating specific haptic interfaces.