The causes of conflict among users of outdoor recreation resources have received little attention from recreation researchers. Knowledge of factors responsible for conflict might assist recreation planners' attempts to reduce future instances of conflict and help management focus its conflict resolution efforts. A theory of conflict is offered as the first step in systematically procuring such knowledge. A definition and characteristics of outdoor recreation conflicts are presented; four comprehensive causes of user conflicts are proposed. Ten propositions are used to link these factors to conflict and suggest future research hypotheses. The social psychological dynamics of conflict, as described here, have implications for understanding the sources of user dissatisfaction. In part two, 120 interviews, taken from two conflict situations involving mechanized and nonmechanized forms of recreation, were used to examine the heuristic value of the theory's concepts. A case study format was used for the analysis. The interviews demonstrated a need to distinguish between potential and felt, or experienced, conflict, due to the latter's dependence on a chance social interaction. Nonmechanized users displayed a high conflict potential, indicated by conflict avoidance behavior, which reduced reports of felt conflict. Fewer mechanized users expressed felt conflict. Stereotyping of the opposite group's lifestyle was found in both cases, as was a lack of intergroup communication. A negative evaluation of the other group's lifestyle seems inherent in such stereotypes. Opposing groups sought different outcomes from interacting with a natural environment though backcountry vehicle users showed a more diverse set of interactions than the literature or stereotypes suggest. Users demonstrated possessiveness for a particular recreation place--this orientation may also exist for categories of places such as National Parks. The findings support the contention that differences in lifestyle, modes of experiencing natural environments, and resource specificity are factors responsible for conflict and worthy of future research.