The present study seeks to accomplish three goals: to shed light on the problem of reason in Husserl’s co-inherited philosophical project, to elucidate his transcendental critique of reason, and to present Husserl's idea of reason in its distinctive features. A historical excursus first provides a frame to understand the necessity of a critique of reason, its proper subject-matter, and its function for the project of genuine philosophy. In particular, this historical reflection identifies the form that a critique must assume in order to fulfil its philosophical-scientific task. The focus is then directed at Husserl's methodological recalibration of the problem of reason. Husserl's ‘prinzipielle Kritik’ is elucidated in his transcendental reassessment of the headings ‘reason’ and ‘unreason,’ and is thought in connection to the concept of Selbstbesinnung. Lastly, Husserl’s idea of reason is reconstructed in relation to, and in disambiguation from, the concepts of self-evidence, logos, synthesis, fulfilment, positing, etc. Reason, as teleological rule and structural form of transcendental subjectivity, is clarified in its dependence on, and irreducibility to, the problems of constitution and in light of the question of its objective/subjective character.