Originally described in patients with chronic epilepsy, nonconvulsive seizures (NCSs) are being recognized with increasing frequency, both in ambulatory patients with cognitive change, and even more so in the critically ill. In fact, the majority of seizures that occur in the critically ill are nonconvulsive and can only be diagnosed with EEG monitoring. The semiology of NCSs and the associated EEG findings are quite variable. There are a number of periodic, rhythmic or stimulation-related EEG patterns in the critically ill of unclear significance and even less clear treatment implications. The field struggles to develop useful diagnostic criteria for NCSs, to standardize nomenclature for the numerous equivocal patterns, and to devise studies that will help determine which patterns should be treated and how aggressively. This review surveys the evidence for and against NCSs causing neuronal injury, and attempts to develop a rational approach to the diagnosis and management of these seizures, particularly in the encephalopathic population.