This work is concerned with the nature and scope of ideology. It begins with an examination of the relationship between theory and practice in the relatively uncontentious area of education. An analysis of different kinds of educational theory reveals the nature of the most comprehensive form of such theorising to be problematic. It is argued that a solution to this problem depends upon the solution to the wider problem of the nature of political ideology, to which the discussion therefore shifts. Existing theories of political ideology being deemed inadequate, a fresh start is made by showing how this form of theory combines the descriptive and the evaluative in a particular way and with particular logical consequences. These consequences characterise ideology as a peculiar form of ethical understanding, involving a distinctive way of thinking and having a logical structure of its own. However, these characteristics which make ideology distinctive are not inherently political, and the possibility of there being several different forms of ideology is discussed. With these conclusions it becomes possible to return to the problem of comprehensive educational theory and show that it is composed of a number of forms of ideology, some related to political ideology and some not. The work concludes with some reflections upon the extent to which ideology is an inevitable component of all thinking about human affairs.