A modified interleukin 15 (IL-15) polypeptide comprising at least one unnatural amino acid covalently attached to a conjugating moiety, wherein the conjugating moiety comprises a water-soluble polymer; wherein the modified IL-15 polypeptide comprises at least about 80% sequence identity to SEQ ID NO: 1; wherein the at least one unnatural amino acid comprises an azido group; and wherein: (a) the position of the at least one unnatural amino acid is selected from L25, N77, V49, and S83, wherein the residue positions correspond to the positions as set forth in SEQ ID NO: 1, and wherein the at least one unnatural amino acid is at a position on the polypeptide that enhances half-life, wherein the enhanced half-life is compared to a half-life of a wild-type IL-15 protein; or (b) the position of the at least one unnatural amino acid is selected from L25, E46, and E53, wherein the residue positions correspond to the positions as set forth in SEQ ID NO: 1, and wherein the at least one unnatural amino acid is at a position on the polypeptide that reduces binding between the modified IL-15 polypeptide and interleukin 15 receptor α (IL-15Rα) but retains significant binding with interleukin 15 receptor βγ (IE-15Rβγ) signaling complex to form an IL-15/IL-15Rβγ complex, wherein the reduced binding to IL-15Rα is compared to binding between a wild-type IL-15 polypeptide and IL-15Rα. wherein the conjugating moiety comprises a water-soluble polymer; wherein the modified IL-15 polypeptide comprises at least about 80% sequence identity to SEQ ID NO: 1; wherein the at least one unnatural amino acid comprises an azido group; and wherein: (a) the position of the at least one unnatural amino acid is selected from L25, N77, V49, and S83, wherein the residue positions correspond to the positions as set forth in SEQ ID NO: 1, and wherein the at least one unnatural amino acid is at a position on the polypeptide that enhances half-life, wherein the enhanced half-life is compared to a half-life of a wild-type IL-15 protein; or (b) the position of the at least one unnatural amino acid is selected from L25, E46, and E53, wherein the residue positions correspond to the positions as set forth in SEQ ID NO: 1, and wherein the at least one unnatural amino acid is at a position on the polypeptide that reduces binding between the modified IL-15 polypeptide and interleukin 15 receptor α (IL-15Rα) but retains significant binding with interleukin 15 receptor βγ (IE-15Rβγ) signaling complex to form an IL-15/IL-15Rβγ complex, wherein the reduced binding to IL-15Rα is compared to binding between a wild-type IL-15 polypeptide and IL-15Rα. (a) the position of the at least one unnatural amino acid is selected from L25, N77, V49, and S83, wherein the residue positions correspond to the positions as set forth in SEQ ID NO: 1, and wherein the at least one unnatural amino acid is at a position on the polypeptide that enhances half-life, wherein the enhanced half-life is compared to a half-life of a wild-type IL-15 protein; or (b) the position of the at least one unnatural amino acid is selected from L25, E46, and E53, wherein the residue positions correspond to the positions as set forth in SEQ ID NO: 1, and wherein the at least one unnatural amino acid is at a position on the polypeptide that reduces binding between the modified IL-15 polypeptide and interleukin 15 receptor α (IL-15Rα) but retains significant binding with interleukin 15 receptor βγ (IE-15Rβγ) signaling complex to form an IL-15/IL-15Rβγ complex, wherein the reduced binding to IL-15Rα is compared to binding between a wild-type IL-15 polypeptide and IL-15Rα.