Mongolia was one of the first countries in the Western Pacific Region to implement IMCI (Integrated Management of Childhood Illness) and to successfully take it to scale. In particular, the approached used in Mongolia resulted in a common understanding by all stakeholders of the need for IMCI and the implications for systematic and strong support from the health system. Subsequently, IMCI has been successfully incorporated into routine national and subnational planning for child health, making it more likely to be sustainable in the longer term. This document reviews the process of IMCI implementation in Mongolia. It is hoped that the experiences and lessons learnt are relevant to all countries introducing IMCI or planning further expansion.