An isolated antibody or antigen-binding fragment thereof that binds to cell surface-expressed human prolactin receptor (PRLR) and blocks prolactin-mediated signaling in cells expressing human PRLR, wherein the antibody or antigen-binding fragment thereof blocks prolactin-mediated signaling in cells expressing human PRLR with an IC 50 of less than about 1.3 nM, as measured using a prolactin signaling blocking assay, wherein: (i) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 292, SEQ ID NO: 294, SEQ ID NO: 296, SEQ ID NO: 300, SEQ ID NO: 302, and SEQ ID NO: 304; (ii) the antibody or antigen-binding fragment comprises complementarity determining regions (CDRs) having the sequences of SEQ ID NO: 4, SEQ ID NO: 6, SEQ ID NO: 8, SEQ ID NO: 12, SEQ ID NO: 14, and SEQ ID NO: 16; (iii) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 20, SEQ ID NO: 22, SEQ ID NO: 24, SEQ ID NO: 28, SEQ ID NO: 30, and SEQ ID NO: 32; (iv) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 52, SEQ ID NO: 54, SEQ ID NO: 56, SEQ ID NO: 60, SEQ ID NO: 62, and SEQ ID NO: 64; (v) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 100, SEQ ID NO: 102, SEQ ID NO: 104, SEQ ID NO: 108, SEQ ID NO: 110, and SEQ ID NO: 112; (vi) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 116, SEQ ID NO: 118, SEQ ID NO: 120, SEQ ID NO: 124, SEQ ID NO: 126, and SEQ ID NO: 128; (vii) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 132, SEQ ID NO: 134, SEQ ID NO: 136, SEQ ID NO: 140, SEQ ID NO: 142, and SEQ ID NO: 144; (viii) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 196, SEQ ID NO: 198, SEQ ID NO: 200, SEQ ID NO: 204, SEQ ID NO: 206, and SEQ ID NO: 208; (ix) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 212, SEQ ID NO: 214, SEQ ID NO: 216, SEQ ID NO: 220, SEQ ID NO: 222, and SEQ ID NO: 224; (x) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 228, SEQ ID NO: 230, SEQ ID NO: 232, SEQ ID NO: 236, SEQ ID NO: 238, and SEQ ID NO: 240; (xi) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 260, SEQ ID NO: 262, SEQ ID NO: 264, SEQ ID NO: 268, SEQ ID NO: 270, and SEQ ID NO: 272; (xii) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 276, SEQ ID NO: 278, SEQ ID NO: 280, SEQ ID NO: 284, SEQ ID NO: 286, and SEQ ID NO: 288; (xiii) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 356, SEQ ID NO: 358, SEQ ID NO: 360, SEQ ID NO: 364, SEQ ID NO: 366, and SEQ ID NO: 368; or (xiv) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 372, SEQ ID NO: 374, SEQ ID NO: 376, SEQ ID NO: 380, SEQ ID NO: 382, and SEQ ID NO: 384. (i) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 292, SEQ ID NO: 294, SEQ ID NO: 296, SEQ ID NO: 300, SEQ ID NO: 302, and SEQ ID NO: 304; (ii) the antibody or antigen-binding fragment comprises complementarity determining regions (CDRs) having the sequences of SEQ ID NO: 4, SEQ ID NO: 6, SEQ ID NO: 8, SEQ ID NO: 12, SEQ ID NO: 14, and SEQ ID NO: 16; (iii) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 20, SEQ ID NO: 22, SEQ ID NO: 24, SEQ ID NO: 28, SEQ ID NO: 30, and SEQ ID NO: 32; (iv) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 52, SEQ ID NO: 54, SEQ ID NO: 56, SEQ ID NO: 60, SEQ ID NO: 62, and SEQ ID NO: 64; (v) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 100, SEQ ID NO: 102, SEQ ID NO: 104, SEQ ID NO: 108, SEQ ID NO: 110, and SEQ ID NO: 112; (vi) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 116, SEQ ID NO: 118, SEQ ID NO: 120, SEQ ID NO: 124, SEQ ID NO: 126, and SEQ ID NO: 128; (vii) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 132, SEQ ID NO: 134, SEQ ID NO: 136, SEQ ID NO: 140, SEQ ID NO: 142, and SEQ ID NO: 144; (viii) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 196, SEQ ID NO: 198, SEQ ID NO: 200, SEQ ID NO: 204, SEQ ID NO: 206, and SEQ ID NO: 208; (ix) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 212, SEQ ID NO: 214, SEQ ID NO: 216, SEQ ID NO: 220, SEQ ID NO: 222, and SEQ ID NO: 224; (x) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 228, SEQ ID NO: 230, SEQ ID NO: 232, SEQ ID NO: 236, SEQ ID NO: 238, and SEQ ID NO: 240; (xi) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 260, SEQ ID NO: 262, SEQ ID NO: 264, SEQ ID NO: 268, SEQ ID NO: 270, and SEQ ID NO: 272; (xii) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 276, SEQ ID NO: 278, SEQ ID NO: 280, SEQ ID NO: 284, SEQ ID NO: 286, and SEQ ID NO: 288; (xiii) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 356, SEQ ID NO: 358, SEQ ID NO: 360, SEQ ID NO: 364, SEQ ID NO: 366, and SEQ ID NO: 368; or (xiv) the antibody or antigen-binding fragment comprises CDRs having the sequences of SEQ ID NO: 372, SEQ ID NO: 374, SEQ ID NO: 376, SEQ ID NO: 380, SEQ ID NO: 382, and SEQ ID NO: 384.