Refractory anemia with ring sideroblasts (RARS) is a subtype of myelodysplastic syndrome (MDS) characterized by 15% or more ring sideroblasts in the bone marrow according to the WHO classification. After Perls staining, ring sideroblasts are defined as erythroblasts in which there are 5 or more siderotic granules covering at least a third of the nuclear circumference. The iron deposited in perinuclear mitochondria of ring sideroblasts is present in the form of mitochondrial ferritin. The molecular basis of MDS with ring sideroblasts has remained unknown until recently. In 2011, whole exome sequencing studies revealed somatic mutations of SF3B1, a gene encoding a core component of RNA splicing machinery, in myelodysplasia with ring sideroblasts. The close relationship between SF3B1 mutation and ring sideroblasts is consistent with a causal relationship, and makes SF3B1 the first gene to be associated with a specific morphological feature in MDS. RARS is mainly characterized by isolated anemia due to ineffective erythropoiesis, and its clinical course is generally benign, although there is a tendency to worsening of anemia in most patients over time. By contrast, refractory cytopenia with multilineage dysplasia and ring sideroblasts (RCMD-RS) is characterized by pancytopenia and dysplasia in two or more myeloid cell lineages. More importantly, patients with RCMD-RS have a higher risk of developing bone marrow failure or progressing to acute myeloid leukemia (AML). Refractory anemia with ring sideroblasts (RARS-T) associated with marked thrombocytosis is a myelodysplastic/myeloproliferative neoplasm associated with both SF3B1 and JAK2 or MPL mutations. RARS-T may develop from an SF3B1 mutated RARS through the acquisition of a JAK2 or MPL mutations in a subclone of hematopoietic cells.