In this issue of Critical Care Clinics, guest editors Drs. Robert M. Kliegman and Brett J. Bordini bring their considerable expertise to the topic of Undiagnosed and Rare Diseases in Critical Care. Top experts in the field cover key topics such as critical genetic arrhythmia disorders, uncommon causes of rhabdomyolysis,  status epilepsy syndromes, autoimmune encephalitis, rapid-onset paralysis and weakness, and more. Contains 17 relevant, practice-oriented topics including understanding cognitive diagnostic errors in the ICU; rapid WES/WGS in the ICU; diagnostic time-outs to improve diagnosis; when "sepsis? is not sepsis: MAS, HLH, malignancies and other sepsis mimics; all that wheezes is not asthma or bronchiolitis; and more.  Provides in-depth clinical reviews on undiagnosed and rare diseases in critical care, offering actionable insights for clinical practice.  Presents the latest information on this timely, focused topic under the leadership of experienced editors in the field. Authors synthesize and distill the latest research and practice guidelines to create clinically significant, topic-based reviews.