Arthritis is the most common joint disease. It impairs patients quality of life on account of the associated chronic pain and loss of joint function. The thumb is the most important digit of the hand and trapeziometacarpal osteoarthritis (carpometacarpal arthritis / CMC-1 arthritis) may significantly compromise functions of the entire hand. CMC-1 arthritis produces several non-specific symptoms, affecting mainly postmenopausal women. The risk of developing CMC-1 arthritis increases with age.Considering these facts, knowledge about the etiopathogenesis and diagnosis of CMC-1 arthritis should be widely disseminated and based on evidence-based medicine. The first step in the diagnostic work-up is a detailed history and clinical examination where the use of more sensitive tests than the grind test, e.g. the pressure-shear test, is recommended. It is advisable to widen the classic radiographic views with additional thumb projections such as Roberts view. The use of magnetic resonance imaging or computed tomography is only advised in special individual cases.This paper aims to present the most up-to-date knowledge about: (1) the anatomy and biomechanics of the trapeziometacarpal joint, (2) the epidemiology of CMC-1 arthritis and (3) its diagnosis. It is based on the latest literature (mainly works published in the last 5 years) acquired from databases such as PubMed, Clinical Key and Science Direct. The article is the first of a two-part series that presents a diagnostic-therapeutic algorithm for CMC-1 arthritis. The authors believe that it may contribute to broadening knowledge about CMC-1 arthritis, optimizing the therapeutic process and improving care for patients with CMC-1 arthritis in Poland.