The thesis deals with the fundamental philosophical question: "What is evil?" It is devided into three main chapters. In the first one I try to verify and define the term "evil". Then I examine two selected concepts dualism and privation theory in the second chapter. While the dualististic theory (which says there are two equal forces) seems to be inconsistent, the privation theory can in some way provide the explanation (or at least partial explanation) of the phenomenon of evil. With these consderations we can approach to the third part where "the argument of evil" (which is usually used as evidence against the exstence of God) is concerned and then questioned.