A global measurement of Crohn's disease activity, comprising clinical, endoscopic, biochemical and pathological features is not available yet and perhaps is unobtainable. In this review we analyse the most used and validated clinical indices (Crohn's Disease Activity Index [CDAI], Perianal Disease Activity Index [PDAI], fistula drainage assessment), quality of life scores (Inflammatory Bowel Disease Questionnaire [IBDQ]), sub-clinical markers (C-reactive protein, faecal calprotectin, intestinal permeability) and endoscopic indices (Crohn's Disease Endoscopic Index of Severity [CDEIS]/Simple Endoscopic Score for Crohn's Disease [SES-CD], Rutgeeerts' score for postsurgical recurrence). We also review the main advantages and disadvantages of each of these scoring systems. All these indices are rather complex and time-consuming, therefore their use is limited to clinical trials. In everyday clinical practice most gastroenterologists rely on their global clinical judgement, which is less reproducible, but simpler for decision-making.