The American College of Physicians (ACP) developed this guideline to provide clinical recommendations on the diagnosis and management of acute left-sided colonic diverticulitis in adults. This guideline is based on current best available evidence about benefits and harms, taken in the context of costs and patient values and preferences. The ACP Clinical Guidelines Committee (CGC) developed this guideline based on a systematic review on the use of computed tomography (CT) for the diagnosis of acute left-sided colonic diverticulitis and on management via hospitalization, antibiotic use, and interventional percutaneous abscess drainage. The systematic review evaluated outcomes that the CGC rated as critical or important. This guideline was developed using the GRADE (Grading of Recommendations Assessment, Development and Evaluation) methodology.