Increasing evidence has revealed the importance of IL-23, which closely resembles IL-12 both structurally and immunologically, in linking innate and adaptive immunity. IL-23, produced by activated type 1 macrophages and dendritic cells (DC), possesses unique roles in the differentiation and expansion of memory T cells. IL-23 has been associated with several inflammatory diseases such as rheumatoid arthritis, inflammatory bowel disease (IBD) and Helicobacter pylori associated gastritis, mainly due to its capacity to induce a strong Th1 type immune response. IL-23 is also associated with Th17 responses and the cytokine produced by the antigen presenting cells (APC), i.e. IL-12 vs IL-23 determines in part if a response is Th1 or Th17. Recent studies have also associated chronic inflammatory diseases such as IBD, psoriasis and myocardial infarction with polymorphisms of the IL-23 receptor complex. The current review focuses on the immunological role of IL-23 and possible therapeutic avenues for inflammatory diseases.