Abrupt changes in ecological systems (ACES)–changes that are fast in time or fast relative to their drivers–are ubiquitous and increasing in frequency as rates of environmental change accelerate. However, ecology still struggles to predict when, where, and why abrupt and fundamental changes are likely to occur in ecosystems. Powerful theoretical frameworks have been developed, yet applying them in real-world landscapes to detect, explain, and anticipate sudden ecosystem changes has proven challenging for several reasons: ecosystems respond to multiple, interacting forcings and disturbances; thresholds are difficult to identify before they have been passed; ecosystems include built-in time lags due to long-lived organisms and slow internal processes; and feedbacks and spatial variation can dampen or amplify ecosystem changes. Extreme events also can overcome mechanisms of ecological resilience and produce abrupt change, yet such events are infrequent by definition and thus difficult to study. Progress in understanding ACES requires integration of theory and high-quality empirical data drawn from a diverse array of managed and unmanaged ecosystems. Recent progress by researchers studying ACES suggests a timely opportunity to synthesize the current state of the science. This organized oral session will begin with presentation of a conceptual framework for understanding how changing drivers and their interactions can produce abrupt change. The introductory talk will be followed by well-developed empirical studies of ACES in diverse ecosystems, including social-ecological systems, and across a range of spatial and temporal scales. Speakers will consider fundamental questions regarding their system, including: What constitutes observational evidence for past or present abrupt change? Which abrupt changes are most important? How are the drivers of abrupt change varying over time? What data and methods are required to diagnose the abrupt changes? What feedbacks stabilize new states, and are the changes persistent (i.e., regime shift)? What forces have the potential to reverse the abrupt change? The session will conclude with a synthetic, prospective talk that integrates theory and data and suggests a research agenda for enhancing knowledge about ACES.