A bispecific antibody comprising an anti-CD3 arm and an anti-HER2 arm, wherein: (a) the anti-CD3 arm comprises a first binding domain comprising: (i) a VH domain comprising an amino acid sequence having at least 95% sequence identity to the amino acid sequence of SEQ ID NO: 184, wherein the VH domain comprises an HVR-H1 comprising the amino acid sequence of SEQ ID NO: 1, an HVR-H2 comprising the amino acid sequence of SEQ ID NO: 2, and an HVR-H3 comprising the amino acid sequence of SEQ ID NO: 3, and (ii) a VL domain comprising an amino acid sequence having at least 95% sequence identity to the amino acid sequence of SEQ ID NO: 185, wherein the VL domain comprises an HVR-L1 comprising the amino acid sequence of SEQ ID NO: 4, an HVR-L2 comprising the amino acid sequence of SEQ ID NO: 5, and an HVR-L3 comprising the amino acid sequence of SEQ ID NO: 6; and (b) the anti-HER2 arm comprises a second binding domain comprising: (i) a VH domain comprising an amino acid sequence having at least 95% sequence identity to the amino acid sequence of SEQ ID NO: 270, wherein the VH domain comprises an HVR-H1 comprising the amino acid sequence of SEQ ID NO: 169, an HVR-H2 comprising the amino acid sequence of SEQ ID NO: 170, and an HVR-H3 comprising the amino acid sequence of SEQ ID NO: 171, and (ii) a VL domain comprising an amino acid sequence having at least 95% sequence identity to the amino acid sequence of SEQ ID NO: 271, wherein the VL domain comprises an HVR-L1 comprising the amino acid sequence of SEQ ID NO: 172, an HVR-L2 comprising the amino acid sequence of SEQ ID NO: 173, and an HVR-L3 comprising the amino acid sequence of SEQ ID NO: 174. (a) the anti-CD3 arm comprises a first binding domain comprising: (i) a VH domain comprising an amino acid sequence having at least 95% sequence identity to the amino acid sequence of SEQ ID NO: 184, wherein the VH domain comprises an HVR-H1 comprising the amino acid sequence of SEQ ID NO: 1, an HVR-H2 comprising the amino acid sequence of SEQ ID NO: 2, and an HVR-H3 comprising the amino acid sequence of SEQ ID NO: 3, and (ii) a VL domain comprising an amino acid sequence having at least 95% sequence identity to the amino acid sequence of SEQ ID NO: 185, wherein the VL domain comprises an HVR-L1 comprising the amino acid sequence of SEQ ID NO: 4, an HVR-L2 comprising the amino acid sequence of SEQ ID NO: 5, and an HVR-L3 comprising the amino acid sequence of SEQ ID NO: 6; and (i) a VH domain comprising an amino acid sequence having at least 95% sequence identity to the amino acid sequence of SEQ ID NO: 184, wherein the VH domain comprises an HVR-H1 comprising the amino acid sequence of SEQ ID NO: 1, an HVR-H2 comprising the amino acid sequence of SEQ ID NO: 2, and an HVR-H3 comprising the amino acid sequence of SEQ ID NO: 3, and (ii) a VL domain comprising an amino acid sequence having at least 95% sequence identity to the amino acid sequence of SEQ ID NO: 185, wherein the VL domain comprises an HVR-L1 comprising the amino acid sequence of SEQ ID NO: 4, an HVR-L2 comprising the amino acid sequence of SEQ ID NO: 5, and an HVR-L3 comprising the amino acid sequence of SEQ ID NO: 6; and (b) the anti-HER2 arm comprises a second binding domain comprising: (i) a VH domain comprising an amino acid sequence having at least 95% sequence identity to the amino acid sequence of SEQ ID NO: 270, wherein the VH domain comprises an HVR-H1 comprising the amino acid sequence of SEQ ID NO: 169, an HVR-H2 comprising the amino acid sequence of SEQ ID NO: 170, and an HVR-H3 comprising the amino acid sequence of SEQ ID NO: 171, and (ii) a VL domain comprising an amino acid sequence having at least 95% sequence identity to the amino acid sequence of SEQ ID NO: 271, wherein the VL domain comprises an HVR-L1 comprising the amino acid sequence of SEQ ID NO: 172, an HVR-L2 comprising the amino acid sequence of SEQ ID NO: 173, and an HVR-L3 comprising the amino acid sequence of SEQ ID NO: 174. (i) a VH domain comprising an amino acid sequence having at least 95% sequence identity to the amino acid sequence of SEQ ID NO: 270, wherein the VH domain comprises an HVR-H1 comprising the amino acid sequence of SEQ ID NO: 169, an HVR-H2 comprising the amino acid sequence of SEQ ID NO: 170, and an HVR-H3 comprising the amino acid sequence of SEQ ID NO: 171, and (ii) a VL domain comprising an amino acid sequence having at least 95% sequence identity to the amino acid sequence of SEQ ID NO: 271, wherein the VL domain comprises an HVR-L1 comprising the amino acid sequence of SEQ ID NO: 172, an HVR-L2 comprising the amino acid sequence of SEQ ID NO: 173, and an HVR-L3 comprising the amino acid sequence of SEQ ID NO: 174.