Disclosed are methods and compositions for inhibiting the growth of a tumor (e.g., a malignant tumor) in a subject. In particular, combination therapies for treating a tumor in a subject by co-administering an agent selected from i) an effective amount of an anti-estrogen agent; ii) an effective amount of a receptor tyrosine kinase inhibitor; iii) an effective amount of a MEK/PI3 kinase/AKT inhibitor; iv) an effective amount of MM-151; v) an effective amount of an mTOR inhibitor; and/or vi) an effective amount of trastuzumab or TMD1, and/or combinations thereof; and an effective amount of a bispecific anti-ErbB2/anti-ErbB3 antibody. Also disclosed is a bispecific anti-ErbB2/anti-ErbB3 antibody for use in the therapy of a tumor in combination with an agent selected from i) an effective amount of an anti-estrogen agent; ii) an effective amount of a receptor tyrosine kinase inhibitor; iii) an effective amount of a MEK/PI3 kinase/AKT inhibitor; iv) an effective amount of MM-151; v) an effective amount of an mTOR inhibitor; and/or vi) an effective amount of trastuzumab or TMD1, and/or combinations thereof.