Since experimental studies with microsatellite instability (MSI)-positive and -negative cell lines as well as clinical data suggest different responses to chemotherapeutic agents, the assessment of MSI status of the tumors is important. It also serves as a screening method for tumors of the hereditary non-polyposis colorectal cancer (HNPCC) spectrum. In cases of sporadic colorectal cancer there is a growing body of evidence indicating the improved prognosis associated with this group of tumors. Furthermore, failure to recognise familial cancer will have a negative impact on the patient's and family's management. Melecular testing for MSI is relatively expensive, however, a combination with immunohisto-chemical (IHC) detection of mismatch repair defects provides some relief to the cost as IHC is relatively inexpensive, fast and technically simple. This book edition of Disease Markers is focused on both the disease manifestations as well as the recent revised Bethesda Guidelines (18, 19) for the diagnosis and decision for genetic testing that will eventually lead to better understanding of the Lynch syndrome (HNPCC) and the MSI phenotype.