This paper offers a new answer to an old question. Others have argued that exploitation is wrong because it is coercive, or degrading, or fails to protect the vulnerable. But these answers only work for certain cases; counterexamples are easily found. In this paper I identify a different answer to the question by placing exploitation within the larger family of wrongs to which it belongs. Exploitation is one species of wrongful gain, and exploiters always gain at the expense of others (...) by inflicting relative losses on disadvantaged parties. They do harm to their victims, even when their interactions are mutually advantageous, by failing to benefit the disadvantaged party as fairness requires. This failure is the essential wrong in every case of wrongful exploitation. At the end of the paper I assess how wrong this failure is as a way to gain at another's expense. (shrink)