<p>The appendix is a small, worm-like diverticulum of the caecum, potentially having a role in regulating intestinal microbiota and immunology. Inflammation of the appendix, acute appendicitis, is one of the most common reasons for acute abdominal pain in children and adults and surgical emergency visits worldwide. The pathophysiology of appendicitis is still poorly understood. During the past decade, evidence has overturned the long-lasting dogma that all appendicitis cases have a clinical course inevitably progressing to perforation and life-threatening peritonitis unless operated upon in a timely manner. Instead, this natural course occurs only in a smaller proportion of patients, for whom emergency appendectomy remains mandatory. Advances in diagnostic accuracy following utilization of clinical scoring systems and imaging has enabled more accurate pre-interventional assessment of appendicitis disease severity. While some patients still require urgent surgery, the majority can be treated successfully with antibiotics, and in some the disease has even been shown to resolve spontaneously. This has confirmed the notion of at least two different forms of appendicitis: non-perforating and perforating, often referred to as uncomplicated and complicated appendicitis. Unified definitions of these forms are still undergoing rigorous research and debate, hampering both comparison of different studies and the establishment of unified treatment guidelines. The current knowledge on the safe and effective outcomes of non-operative treatment alternatives has further underlined the need for standardized uniform definitions of appendicitis severity and assessment of the success of two fundamentally different treatment options.</p>