Tarski's most influential paper has been edited a number of times in multiple languages. Each new edition has become an improved version of the previous one. Misprints, as well as some translation errors, have been corrected; sometimes whole passages have been clarified and rewritten. A thorough revision of the whole article, however, was never possible. Due to the number of editions, it is crucial that I indicate the copies used here. For this commentary the English edition (Tarski 2006) by John Corcoran was used and the English quotes refer to this edition, except when otherwise stated. In exceptional cases I will be referring to the edition (Tarski 1956) always indicating it explicitly.