In 1980, the World Health Organisation (WHO) ended a long phase of confusion by providing international standards for diagnosis and classification of diabetes . Before this, confusion existed with respect to the glucose threshold for diagnosis of diabetes and other categories of glucose intolerance as well as the glucose load used for the oral glucose tolerance test. As always, however, new scientific data and insight combined with health political issues have led to several revisions of the diagnostic criteria and classification of patients with diabetes as well as with other categories of glucose intolerance. The first revision was made in 1985 , the second in 1999  and most recently the third revision came out in 2006  based on a collaborative effort between WHO and the International Diabetes Federation (IDF). In addition to these global definitions, national agencies like the American Diabetes Association (ADA) [5,6] as well as international organizations such as the IDF  have provided definitions that are not fully in accordance with the WHO definitions of diabetes, glucose intolerance and the metabolic syndrome (Table 1). This lack of concordance has not only created confusion among researchers but also among clinicians. As a consequence of the use of different diagnostic criteria, studies and trials may no longer be directly comparable as "diabetes" "IGT" or "IFG" no longer represents the same population in different studies. Finally, the fact that leading personalities within the field of diabetes have identified themselves with some definitions and not with others as the "fathers and mothers" of the different definitions has split observers and users into groups of "believers" rather than into scientific orientation. This chapter focuses on the following questions related to definition and classification of diabetes: •What are the criteria used to identify diagnostic thresholds for DM and impaired glucose regulation (IFG and IGT together)? •Redefining diabetes-what are the consequences for prognosis and diagnostic tests? •Reclassifying diabetes-how to differentiate between type 1 and type 2 diabetes? •Establishing a third category-IFG-why and what is IFG? •Lowering the threshold for IFG-what are the consequences? •Open questions by 2007.