2D semiconductor quantum wells have been recognized as potential candidates for various quantum devices. In quantum wells, electrons and holes are spatially confined within a finite thickness and freely move in 2D space. Much effort has focused on shape control of colloidal semiconductor nanocrystals (NCs), and synthesis of 2D colloidal NCs has been achieved very recently. Here, recent advances in colloidal synthesis of uniform and ultrathin 2D CdSe NCs are highlighted. Structural and optical property characterization of these quantum-sized 2D CdSe NCs is discussed. Additionally, 2D CdSe NCs doped with Mn2+ ions for dilute magnetic semiconductors (DMS) are presented. These 2D CdSe-based NCs can be used as model systems for studying quantum-well structures.