G. W. Leibniz is noted for his metaphysical optimism, that is, his belief that this is the best possible world, chosen and created among all the others by God because it possesses the highest degree of perfection. However, the main objection to this belief concerns the existence of evil. How did evil come into the world if this is the best of all possible worlds? The answer to the question, according to Leibniz, lies in the fact that there are three different types of evil: metaphysical, physical and moral. Leibniz argues that God created this world by the principle of the best, claiming that if the world lacked only a part of the existing evil, it would not be the best possible world anymore because its harmony would be disrupted. The aim of this paper is to critically expose the problem of evil in the philosophy of G. W. Leibniz. For that reason his fundamental writings, such as Theodicy and the Monadology are discussed, as well as the relevant secondary literature, including encyclopaedias and dictionaries of philosophy.