This chapter presents discussion on immunosuppressants and immunostimulants. Immunosuppressants are classified into calcineurin inhibitors/T-cell inhibitors, antiproliferative agents, cytotoxic drugs, glucocorticoids, and immunosuppressive antibodies which are described broadly in this chapter. Immunostimulants Increase immune response in immunodeficiency patients, e.g. acquired immune deficiency syndrome (AIDS), cancers, chronic illness, etc. Drugs used as immunostimulants are bacillus Calmette–Guérin vaccine, levamisole (anthelmintic), thalidomide, interferons, and thymosin. Immunizing agents are vaccines, immunoglobulins and antisera. They are used for immunization against bacterial and viral infections.