Hematological traits are essential biomedical indicators that are widely used in clinical practice. The elucidation of the etiology that determines an individual's hematological traits would have a substantial impact. Hematological traits are known to be heritable, and it has been suggested that genetic factors contribute significantly to the inter-individual variance of these traits. Here, we review our current knowledge regarding the genetic architecture of hematological traits in humans, most of which has been obtained through recent developments in genome-wide association studies (GWAS). In addition to current knowledge, which is based on the hematological traits of the three major blood-cell lineages (white blood cells; WBC, red blood cells; RBC, and platelets; PLT), we propose future approaches that would be useful as a next step in the post-GWAS era.