This chapter presents a state of the art summary of myelodysplastic syndromes (MDS) to its definition, biology, epidemiology, disease mechanisms, clinical signs, diagnosis, treatment and prognosis. MDS are further divided into MDS with dysplasia, MDS with ring sideroblasts, MDS with excess blasts, MDS with isolated del, MDS unclassifiable, provisional entity—refractory cytopenia of childhood, and myeloid neoplasms with germline predisposition. Approximately 90% of MDS cases arise de novo without identifiable cause, and complex epigenetic, genetic, and immunologic mechanisms appear to underline MDS pathogenesis. MDS is a relatively common hematological malignancy, with an annual incidence of 5 per 100,000 in the general population, 30 per 100,000 in the age group of greater than 60 years, and 50 per 100,000 in the age group of greater than 80 years. Treatment options for MDS consist of hypomethylating agents, lenalidomide, and allogeneic hematopoietic stem cell transplantation.