An imperfect competitor is in equilibrium when the marginal cost (MC) is equal to marginal revenue (MR). This is because a firm will produce at a level where the additional cost of producing one more unit of output (MC) is equal to the additional revenue gained by selling that additional unit (MR). At this point, the firm is maximizing its profits and there is no incentive to produce more or less output. Therefore, the firm will continue to produce at this level of output until there is a change in either cost or revenue.