This Element is an elementary introduction to atheism and agnosticism. It begins with a careful characterisation of atheism and agnosticism, distinguishing them from many other things with which they are often conflated. After a brief discussion of the theoretical framework within which atheism and agnosticism are properly evaluated, it then turns to the sketching of cases for atheism and agnosticism. In both cases, the aim is not conviction, but rather advancement of understanding: the point of the cases is to make it intelligible why some take themselves to have compelling reason to adopt atheism or agnosticism.