Since insulin is the principal hormone that regulates uptake of glucose into most cells from the blood (primarily muscle and fat cells, but not central nervous system cells), deficiency of insulin or the insensitivity of its receptors plays a central role in all forms of diabetes mellitus.

Currently, type 1 diabetes is treated with insulin injections, lifestyle adjustments, and careful monitoring of blood glucose levels using blood test kits. Insulin delivery is also available by an insulin pump, which allows the infusion of insulin 24 hours a day at preset levels, and the ability to program push doses (bolus) of insulin as needed at meal times. The treatment must be continued indefinitely. During life-time treatment which does not impair normal activities if carried out systematically with discipline, the average glucose level for the type I diabetic patient must be at 110 mg/dl - 140 mg/dl as normal although 150 mg/dl is acceptable. Some people prefer an average above 150 mg/dl. 200-250 mg/dl is the middle-range of high blood glucose when discomfort by a need to urinate begins at 170 mg/dl though this is dependent on the individuals' target range. 300-350 mg/dl requires a ketone analysis as well as insulin injections immediately. 350 mg/dl and above can lead to ketoacidosis if not treated with sugar-free liquids or water ideally consumed.

Type 2

Type 2 diabetes is characterized by "insulin resistance" as body cells do not respond appropriately when insulin is present. This is a more complex problem than type 1, but is often easier to treat, since insulin is still produced, especially in the initial years. Type 2 may go unnoticed for years in a patient before diagnosis, since the symptoms are typically milder (no ketoacidosis) and can be sporadic. However, severe complications can result from unnoticed type 2 diabetes, including renal failure, and coronary artery disease.

Type 2 is initially treated by changes in diet and through weight loss. This can restore insulin sensitivity, even when the weight lost is modest e.g. around 5 kg (10 to 15 lb). The next step, if necessary, is treatment with oral antidiabetic drugs: the sulphonylureas, metformin, or (if these are insufficient) thiazolidinediones. If these fail, insulin therapy may be necessary to maintain normal glucose levels. Glucose levels at 140 mg/dl and above determine Type II diabetes in pre-diabetic patients. Any type of diabetes if properly dealt with a disciplined regimen of blood glucose checks will enhance the quality of life in no way allowing diabetes to become a life-threatening disability which can happen if left unchecked. Glucose checks are a necessary part of diabetes management which have to be done in awkward moments. To strive for better control in Type I as well as Type II diabetes, glucose levels must be checked periodically to maintain a high standard of care and quality of life regardless of of the discomfort others may have with the glucose monitoring event.

Gestational diabetes
Main article: Gestational diabetes

Gestational diabetes mellitus appears in about 2-5% of all pregnancies. It is temporary and fully treatable, but if untreated it may cause problems with the pregnancy, including macrosomia (high birth weight) of the child. It requires careful medical supervision during the pregnancy. In addition, about 20-50% of these women go on to develop type 2 diabetes.

Other types

There are several causes of diabetes which do not fit into type 1, type 2, or gestational diabetes

Genetic defects in beta cells.
Genetically related insulin resistance.
Diseases of the pancreas.
Caused by hormonal defects.
Caused by chemicals or drugs.
"Malnutrition-related diabetes mellitus" (MRDM or MMDM) was introduced by the WHO as the third major category of diabetes in the 1980s. However, in 1999, a WHO working group recommended that MRDM be deprecated, and proposed a new taxonomy for alternative forms of diabetes. Classification of non-type 1, non-type 2, non-gestational diabetes remains controversial.

Genetics
Both type 1 and type 2 diabetes are at least partly inherited. Type 1 diabetes appears to be triggered by infection, stress, or environmental factors (e.g. exposure to a causative agent). There is a genetic element in the susceptibility of individuals to some of these triggers which has been traced to particular HLA genotypes (i.e. genetic "self" identifiers used by the immune system). However, even in those who have inherited the susceptibility, type 1 diabetes mellitus seems to require an environmental trigger. A small proportion of type 1 diabetics carry a mutation that causes maturity onset diabetes of the young (MODY).