The term colonic ischaemia was first coined approximately 40 years ago. Up until that point, the disease was only diagnosed in cases where the colon was completely gangrenous. In the 1960s reversible non-transmural ischaemia of the colon was described and much has been written about the disease since. Ischaemic colitis is usually a disease process that is seen in the elderly. However, the true incidence of the disease remains unknown since many cases are probably never diagnosed. We now know that the disease can occur both with occlusive and non-occlusive vascular disease, the latter being much more common. Although factors that contribute to non-occlusive disease have been identified, often the exact pathophysiology is unclear and this is an area where further research is needed. Known facts about the pathophysiology and aetiology of the disease will be discussed in this chapter. The key to diagnosis is often a high degree of clinical suspicion. Various diagnostic modalities are described, but colonoscopy remains the main instrument of accurate diagnosis. Fortunately the majority of patients will have self-limiting disease and, accordingly, they usually respond to conservative therapy. When the disease progresses to transmural infarction, surgery is required and the prognosis is guarded. Since many of the patients are elderly with other underlying diseases, it is unlikely that major treatment advances will allow us to significantly alter the prognosis in this group of patients with transmural ischaemia.