This article argues that more systematic attention needs to be given to academic advice to managers and policy-makers. It explores the 'third world' of academic public management - the giving of advice by academics to politicians and public servants. It first focuses on the question of what kind of advice is given. At least eight different modes of advice are identified, each with rather different implications for the rules of engagement. This is used as a basis for a subsequent exploration of the more debated issue of the 'rules of engagement' between academics and practitioners. Finally, a discussion is opened of how advice work could be assessed for its strictly scientific merit.