This review will discuss the concept of internalizing disorders. It will describe the two main types of internalizing disorder: depressive and anxiety disorders. It will discuss how they have much in common, but that there are also key differences. The review will use data from modern studies of symptom factor analysis, aetiology, treatment and prognosis to illustrate the commonalities and differences. It will conclude by trying to answer where internalizing disorders should be placed in future diagnostic classification schemes.