The publication of Heidegger's Black Notebooks (Schwarze Hefte) has provoked a storm of controversy. Much of this has centred on the pro-Nazi and anti-Semitic comments the volumes contain. But these aspects of the Notebooks are perhaps the least surprising and important. This essay offers a summary overview of the issues to which the Notebooks give rise, at the same time as it also aims to provide a preliminary assessment of their overall significance, especially in relation to what they show about the nature and development of Heidegger's thinking from the early 1930s to the late 1940s.