This paper concerns the recent debate on the nature and motivations of the epistemological project advanced in Rudolf Carnap’s (1891-1970) Aufbau. Much of this debate has been initiated by Michael Friedman (1999) and Alan Richardson (1998) who argue, against the received view of the Aufbau as a foundationalist defense of empiricism, that Carnap’s epistemological project is located in the tradition of neo-Kantian epistemology. On this revisionist reading of the Aufbau, Carnap’s project is not motivated to address traditionally empiricist questions regarding the justification of knowledge, but rather to show how objective knowledge is possible. A central aspect of the Aufbau that is neglected in the revisionists’ analysis is the role of epistemic justification in Carnap’s project. The aim of the present study is to argue that although the nature of the epistemology in the Aufbau is neo-Kantian, Carnap’s method of construction theory (or rational reconstruction) is formulated precisely as a method for the justification of conceptual knowledge. Carnap’s project radically redefines ‘justification’ into a formal-conventional notion, and is part of Carnap’s more general agenda of redefining epistemology as a purely formal discipline.