In 1998, an international and multidisciplinary group of experts (from the fields of spine surgery, physiotherapy, occupational therapy, rheumatology, primary care medicine, internal medicine, health economics and epidemiology) proposed a short multidimensional series of core outcome items for use in patients with low back disorders. In 2005 and 2006, two independent research groups published studies validating these items when combined to form an outcome instrument that was subsequently coined the Core Outcome Measures Index for the back (COMI-back). It now exists in at least 22 languages, is used as the primary outcome measure in numerous research studies, and has been adopted by International and National Surgical and Non-Surgical Spine Registries. Its Minimal Clinically Important Change score for both improvement and worsening have been calculated. It has been shown to be as responsive an outcome measure as many other, long-standing legacy outcome measures (e.g. Oswestry Disability Index-ODI) and various condition-specific instruments used in assessing patients with specific pathologies such as spinal stenosis, spinal deformity, or spinal tumour. A cross-walk to convert COMI scores to ODI scores has been developed. Other musculoskeletal specialties have taken the COMI-back and modified the wording to reflect the joint being enquired about to produce COMI-neck, COMI-knee and COMI-hip versions; these have been subsequently validated for use in patients with the corresponding musculoskeletal disorders. Predictor studies using machine-learning methods and sophisticated statistical analyses have been carried out to predict post-surgical COMI scores, based on a minimal set of baseline characteristics. Part 1 of this review gives an overview of the development of the COMI and its success as a brief but responsive multidimensional outcome instrument over the last 18 years, whilst Part 2 serves to describe its content, scoring, language versions and applications.