This chapter critically evaluates the likelihood that planet Earth will cross one or more global environmental tipping points, resulting in a degraded state that would be difficult to reverse. Ecological tipping points occur when components of a system change rapidly due an initial forcing that is amplified by positive feedbacks, resulting in a regime shift. The chapter examines the evidence in support of biological and geophysical boundaries that clearly delimit a “safe operating space” for people and biodiversity. For individual ecosystems, abrupt state transitions have been documented. However, apart from the climate system, there is scant evidence (or theoretical justification) to support the view that global aggregates like biodiversity, chemical cycles, or resource extraction have planetary thresholds that define the boundaries of a global safe operating space. Acknowledging the absence of clear evidence for thresholds or boundaries at the global level does not diminish the seriousness of anthropogenic impacts. It does, however, imply that local-scale mitigation actions will be most effective.