The worldwide epidemic of obesity is a serious threat to public health, because obesity results in insulin resistance, which leads to metabolic syndrome. Obesity is determined by both adipocyte size (adipocyte hypertrophy) and adipocyte number (adipocyte hyperplasia). Recent studies have begun to clarify the mechanism which regulates the size and number of adipocytes. In addition, chronic inflammation in insulin sensitive tissues receives much attention as the mediator between obesity and insulin resistance. In particular, excess triacylglycerol storage in adipocytes induces macrophage infiltration into adipose tissues, which results in inflammation and alters the expression and secretion of various adipokines. Inflammation pathway is also activated in liver. In addition, myeloid cells are tightly involved in inflammation in adipose tissues and liver. Inflammation in these tissues results in systemic insulin resistance. Intervention which decreases inflammation may be useful treatment for insulin resistance.