Managers of protected areas are under increasing pressure to measure their effectiveness in conserving native biological diversity in ways that are scientifically sound, practical, and comparable among protected areas over time. The Nature Conservancy and its partners have developed a “Measures of Success” framework with four core components: (1) identifying a limited number of focal conservation targets, (2) identifying key ecological attributes for these targets, (3) identifying an acceptable range of variation for each attribute as measured by properly selected indicators, and (4) rating target status based on whether or not the target's key attributes are within their acceptable ranges of variation. A target cannot be considered “conserved” if any of its key ecological attributes exceeds its acceptable range of variation. The framework provides a rigorous basis not only for measuring success but for setting conservation objectives, assessing threats to biodiversity, identifying monitoring and research needs, and communicating management information to nonspecialists.