This paper is intended to provide a source of discussion for a workshop sponsored by the North American Commission for Environmental Cooperation, to be held on 1-3 May 2001 in Monterey, California. The workshop will focus on setting realistic and achievable conservation targets for Pacific MPA’s, and on developing practical and rigorous means of measuring their effectiveness in meeting their objectives. We will first contrast the effects that MPA’s have on the organisms they protect, versus their effectiveness in achieving their stated goals. We then consider the concept of conservation targets and limits, and emphasize the need to explicitly quantify the objectives of a MPA and develop predicted measures of effects that can be used to evaluate effectiveness. We then identify several examples of measures that might be used to evaluate effectiveness of MPA’s in fulfilling three classes of objectives: fisheries, biodiversity, and migratory species conservation. Finally, we explore in more detail the role uncertainty plays in developing effectiveness monitoring programs, and how conclusions drawn from effectiveness assessment are subject to uncertainty at many levels.