An antibody-drug conjugate comprising a drug conjugated to: (i) a monoclonal antibody that binds fibroblast growth factor receptor 4 (FGFR4), or antigen-binding fragment thereof, comprising a variable heavy domain and a variable light domain, wherein: the VH domain of the antibody comprises the complementarity determining region sequences of SEQ ID NO: 15 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 16; the VH domain of the antibody comprises the CDR sequences of SEQ ID NO: 3 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 4; the VH domain of the antibody comprises the CDR sequences of SEQ ID NO: 17 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 18; the VH domain of the antibody comprises the CDR sequences of SEQ ID NO: 7 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 8; the VH domain of the antibody comprises the CDR sequences of SEQ ID NO: 9 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 10 or (ii) a FGFR4-specific monoclonal antibody or antigen-binding fragment comprising a VH domain and a VL domain, wherein the VH domain of the antibody comprises the CDR sequences of SEQ ID NO: 1 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 2, particularly wherein the drug is a small molecule, or wherein the drug is an anti-microtubule agent, an anti-mitotic agent and/or a cytotoxic agent, and/or wherein the drug is monomethyl auristatin F (MMAF) or duocarmycin. (i) a monoclonal antibody that binds fibroblast growth factor receptor 4 (FGFR4), or antigen-binding fragment thereof, comprising a variable heavy domain and a variable light domain, wherein: the VH domain of the antibody comprises the complementarity determining region sequences of SEQ ID NO: 15 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 16; the VH domain of the antibody comprises the CDR sequences of SEQ ID NO: 3 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 4; the VH domain of the antibody comprises the CDR sequences of SEQ ID NO: 17 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 18; the VH domain of the antibody comprises the CDR sequences of SEQ ID NO: 7 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 8; the VH domain of the antibody comprises the CDR sequences of SEQ ID NO: 9 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 10 or (ii) a FGFR4-specific monoclonal antibody or antigen-binding fragment comprising a VH domain and a VL domain, wherein the VH domain of the antibody comprises the CDR sequences of SEQ ID NO: 1 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 2, particularly wherein the drug is a small molecule, or wherein the drug is an anti-microtubule agent, an anti-mitotic agent and/or a cytotoxic agent, and/or wherein the drug is monomethyl auristatin F (MMAF) or duocarmycin. the VH domain of the antibody comprises the complementarity determining region sequences of SEQ ID NO: 15 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 16; the VH domain of the antibody comprises the CDR sequences of SEQ ID NO: 3 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 4; the VH domain of the antibody comprises the CDR sequences of SEQ ID NO: 17 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 18; the VH domain of the antibody comprises the CDR sequences of SEQ ID NO: 7 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 8; the VH domain of the antibody comprises the CDR sequences of SEQ ID NO: 9 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 10 or (ii) a FGFR4-specific monoclonal antibody or antigen-binding fragment comprising a VH domain and a VL domain, wherein the VH domain of the antibody comprises the CDR sequences of SEQ ID NO: 1 and the VL domain of the antibody comprises the CDR sequences of SEQ ID NO: 2, particularly wherein the drug is a small molecule, or wherein the drug is an anti-microtubule agent, an anti-mitotic agent and/or a cytotoxic agent, and/or wherein the drug is monomethyl auristatin F (MMAF) or duocarmycin.