This talk will review the definition, burden, and prognosis of chronic and persistent critical illness. Historically, prolonged ICU stays have been thought of as being synonymous with prolonged mechanical ventilation, which was termed “chronic critical illness”. However, many patients remain stuck in the ICU for reasons other than persistent mechanical ventilation. While some patients have persistent organ failure necessitating ICU care, other patients experience a cascade of problems, such that what keeps them in the ICU may differ substantially from what brought them to the ICU in the first place. A new syndrome of “persistent critical illness” has been defined to encompass the broad scenarios by which patients remain ICU-dependent due to ongoing illness and clinical instability that is no longer directly attributable to the original organ dysfunction prompting ICU admission. In several large-scale epidemiologic studies across multiple countries, persistent critical illness had been empirically determiend to begin around 10 days after ICU admission. This is the point at which characteristics at ICU admission (admission diagnoses and physiologic derangements) are no longer more predictive of mortality than antecedent characteristics (age, sex, chronic health status). While persistent critical illness occurs in just 5% of ICU admissions, it has a disproportionate impact on ICU resource use—accounting for nearly a third of all ICU bed-days. Furthermore, rates of persistent critical illness vary more than 3-fold across hospitals, suggesting the importance of contextual as well as patient factors in the development of persistent critical illness.