The topic of this paper is the epistemological thought of the British philosopher John Locke. This paper set out to provide the basics of Locke's thought as expressed in his work An Essay Concerning Human Understanding. A special emphasis has been placed on the most significant terms of Locke's epistemology, as well as on the way in which Locke used these terms in the development of his epistemology. This paper firstly dealt with the topic of reason in Locke's aforementioned work, followed by his critique of the theory of innate ideas. Moreover, what has also been discussed was his theory of ideas, as well as the distinction between simple and complex ideas, together with the theory of primary and secondary qualities. Furthermore, what has also been mentioned were the basics of Locke's epistemological theory, together with his interpretation of the degrees, validity, extent and reality of human knowledge. Finally, what has been put forth was the overview of the most important theses, along with a critical review of the strengths and weaknesses in Locke's Essay.