The contrast between the critical theory of the Frankfurt School and Karl Mannheim's sociology of knowledge has often been noted, but never fully explicated. Most accounts of the Frankfurt School's critique of Mannheim have taken the form of partisan affirmations or denials of its success. This study provides the first detailed comparison of these two conflicting approaches to the theory of ideology. Following reconstructions of the origins of the two perspectives and a review of the Frankfurt School's critique of Mannheim, a systematic comparison is developed around the four central issues of totality, consciousness and social existence, ideology, and truth. An analysis of the significance of the contrast for current debates in social theory is offered in conclusion.