The nature of scientific progress and the rationality of scientific change lie at the centre of Karl Popper’s and Thomas Kuhn’s thought. This paper provides an analysis of the Popper - Kuhn debate over those issues; according to which, Kuhn is portrayed as subjectivist and relativist, while Popper emerges as objectivist and realist. The paper is divided into three parts. Popper’s claims regarding scientific progress and rationality are examined in Part One. It is argued that Popper’s philosophy is inherently value-driven, while defending the objective characteristics of scientific truth. Part Two explores Kuhn’s conception of science, of the rationality of science and scientific progress. Kuhn argued that knowledge is relative only to the accepted paradigm. Part Three is taken up with a comparative discussion of the main issues related to the Kuhn - Popper debate.