Ecological thresholds are points at which an abrupt change occurs in the condition or functioning of an ecosystem. This concept emerged from the idea that ecosystems often exhibit multiple “stable” states. The analog to ecological thresholds in the physical sciences dates back to the 1700s, when John Dalton described the changing state of water from a liquid to vapor. Certainly not all ecosystems exhibit threshold responses to ecosystem drivers or stressors, but such responses have been frequently found in studies of aquatic systems, wildlife responses to landscape change (e.g., fragmentation and habitat area), wildlife population dynamics, and community responses to invasive species. It is important to consider the possibility of threshold responses in management because the costs and consequences of crossing them may be quite high. Once a threshold is crossed, it may irreversibly shift an ecosystem to an undesirable state, limit the future management options, and force policy choices. However, predicting and preventing threshold responses can be quite challenging due to the inherent complexities that may trigger them and the interactions that result from them.