The present invention provides bispecific antigen-binding molecules having a monovalent arm specific to a first target antigen (e.g., a T cell antigen, such as CD3) and a bivalent arm specific for a second target antigen (e.g., a tumor antigen, such as HER2). Bispecific antigen-binding molecules are useful in the treatment of disorders, such as cancer (e.g., HER2-positive cancer). The invention also features methods of producing bispecific antigen-binding molecules, methods of treating disorders using bispecific antigen-binding molecules, and compositions including bispecific antigen-binding molecules.