Johann Gottlieb Fichte (1762–1814) was a founding figure of the philosophical movement known as German Idealism, which developed from the theoretical and ethical writings of Kant. In this Critical Guide, leading Fichte scholars offer new essays exploring the foundation and hallmark of Fichte's major philosophical achievement, his Wissenschaftslehre or Doctrine of Scientific Knowledge.  Their essays include detailed and accessible analyses of the central features of the Doctrine (epistemology, philosophical method, the nature of rationality, and philosophy of mind) in its Jena and Berlin versions, together with assessments of Fichte's engagement with his contemporaries (Kant, Maimon, Schulze, and Reinhold) and with Spinoza. Other essays chart new directions in Fichte studies, challenge some traditional and orthodox interpretations of aspects of Fichte's work, and show how the Doctrine of Knowledge can be a valuable resource and interlocutor for contemporary philosophical problem-solving.