The focus of my thesis is Epicurean epistemology. Epicurean epistemology is based the thesis according to which all perceptions are true obliging himself to empiricism. However his strong epistemological empiricism is combined with and even defended through atomistic theory, according to which the knowledge of the real structure of the world goes beyond the scope of our experience. In the thesis I shall examine the way Epicurean epistemology solves this problem of incompatibility between empiricism and atomism. Epicurus solution is based on establishing the means by which knowledge can be gained, that is, the criteria of truth. Epicurus held that there are three separate criteria: perceptions, preconceptions and feelings. In the first chapter I introduce the term ‘criterion’ and explore the general context in which the debate about the criterion of truth. I argue that Epicurus’ main motivation lies in the kind of atomistic skepticism according to which phenomenal knowledge is impossible. In the second chapter I explore the first criterion of truth, perceptions, and examine interpretations of the central claim that all perceptions are true. The chapter is divided in two parts since I shall argue that Epicurus offers two different lines of argument to support the thesis that all perceptions are true. The first claims that unless we accept that all perceptions as true, knowledge will be impossible. The second defense the incorrigibility of perception on the basis of atomistic theory of perception (eidolic theory). In the third chapter I explore preconceptions, the second criterion of truth. I argue that the process of formation is purely empirical since they are produced through repeated perceptions of individual instances of a particular type of thing. Given this, preconceptions are the means by which we recognize types of object, and as such are fundamental to Epicurus’ account of how we gain knowledge of things. Besides this function, I shall argue that by preconceptions we engaged in the process of interpretation of perceptual content, its conceptualization which in the end enables us to form beliefs. In the last chapter I discuss Epicurus’ methods of testing beliefs. Beliefs about observable objects are tested by direct perception through the methods of witnessing or nonwitnessing. By establishing methods of counterwitnessing and non-counterwitnessing for testing beliefs about what is hidden from direct perception, Epicurus show the way in which we infer about the existence of atoms and other non-evident things on the grounds of perception.