Calprotectin is a member of the S100 family of calcium binding proteins, being a hetero dimmer of S100A8/9 and accounts for 60% of the protein in the cytoplasm of neutrophils. It is released into faeces when inflammation of the gut is present. Measurement of faecal calprotectin has been used to help differentiate inflammatory bowel disease (IBD) from irritable bowel syndrome (IBS) for nearly 20 years since the first major study carried out at King’s College Hospital of 620 patients attending a gastroenterology outpatient clinic. Since then many studies have been carried out in both adults and children with sensitivities and specificities of more than 80% for distinguishing IBD from IBS being consistently achieved. Faecal calprotectin can be used to help determine the extent of inflammation in established IBD to gauge the need for or response to treatment. Faecal calprotectin correlates well with histological scores of inflammation in IBD and can predict both response to treatment and the risk of relapse. Recent data in patients receiving anti-TNF therapy suggests that faecal calprotectin measured after six weeks of therapy can identify those patients who will respond to this treatment (fall in calprotectin >70%). The potential use of faecal calprotectin in bowel cancer, infectious diarrhoea, intestinal transplantation and graft versus host disease after bone marrow transplantation will be presented.