A conjugate of Formula, or pharmaceutically acceptable salt thereof: wherein: R 50 is where the * indicates the point of attachment to Ab; Ab is an antibody or antigen binding fragment thereof that specifically binds to human HER2; R 1 is -NHR 2 or -NHCHR 2 R 3; R 2 is -C 3 -C 6 alkyl or -C 4 -C 6 alkyl; R 3 is L 1 OH; L 1 is -(OH 2) m -; L 2 is -(CH 2) n -, -((CH 2) n O) t (CH 2) n -, -(CH 2) n X 1 (CH 2) n -, -(OH 2) n NHO(=O)(OH 2) n -, -(CH 2) n NHC(=O)(CH 2) n C(=O)NH(CH 2) n -, -((CH 2) n O) t (CH 2) n NHC(=O)(CH 2) n, -C(=O)(CH 2) n -, -C(=O)((CH 2) n O) t (CH 2) n -, -C(=O)((CH 2) n O) t (CH 2) n X 1 (CH 2) n -, -C(=O)((CH 2) n O) t (CH 2) n NHC(=O)(CH 2) n -, -C(=O)((CH 2) n O) t (CH 2) n C(=O)NH(CH 2) n -, -C(=O)NH((CH 2) n O) t (CH 2) n X 1 (CH 2) n -, -C(=O)X 2 X 3 C(=O)((CH 2) n O) t (CH 2) n -,-C(=O)X 2 X 3 C(=O)(CH 2) n -, -C(=O)X 2 C(=O)(CH 2) n NHC(=O)(CH 2) n -,-C(=O)X 2 C(=O)(CH 2) n NHC(=O)((CH 2) n O) t (CH 2) n -, -C(=O)(CH 2) n C(R 7) 2 -,-C(=O)(CH 2) n C(R 7) 2 SS(CH 2) n NHC(=O)(CH 2) n -, -(CH 2) n X 2 C(=O)(CH 2) n NHC(=O)((CH 2) n O) t (CH 2) n - or -C(=O)(CH 2) n C(=O)NH(CH 2) n; R 40 is -S-,-NHC(=O)CH 2 -, -S(=O) 2 CH 2 CH 2 -, -(CH 2) 2 S(=O) 2 CH 2 CH 2 -, -NHS(=O) 2 CH 2 CH 2,-NHC(=O)CH 2 CH 2 -, -CH 2 NHCH 2 CH 2 -, -NHCH 2 CH 2 -, or X 1 is X 2 is X 3 is each R 7 is independently selected from H and C 1 -C 6 alkyl; each R 8 is independently selected from H, C 1 -C 6 alkyl, F, Cl, and -OH; each R 9 is independently selected from H, C 1 -C 6 alkyl, F, Cl, -NH 2, -OCH 3, -OCH 2 CH 3,-N(CH 3) 2, -CN, -NO 2 and -OH; each R 10 is independently selected from H, C 1-6 alkyl, fluoro, benzyloxy substituted with-C(=O)OH, benzyl substituted with -C(=O)OH, C 1-4 alkoxy substituted with -C(=O)OH and C 1-4 alkyl substituted with -C(=O)OH; R 12 is H, methyl or phenyl; each m is independently selected from 1, 2, 3, and 4; each n is independently selected from 1, 2, 3, and 4; each t is independently selected from 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 and 18, and y is an integer from 1 to 16. R 50 is where the * indicates the point of attachment to Ab; Ab is an antibody or antigen binding fragment thereof that specifically binds to human HER2; R 1 is -NHR 2 or -NHCHR 2 R 3; R 2 is -C 3 -C 6 alkyl or -C 4 -C 6 alkyl; R 3 is L 1 OH; L 1 is -(OH 2) m -; L 2 is -(CH 2) n -, -((CH 2) n O) t (CH 2) n -, -(CH 2) n X 1 (CH 2) n -, -(OH 2) n NHO(=O)(OH 2) n -, -(CH 2) n NHC(=O)(CH 2) n C(=O)NH(CH 2) n -, -((CH 2) n O) t (CH 2) n NHC(=O)(CH 2) n, -C(=O)(CH 2) n -, -C(=O)((CH 2) n O) t (CH 2) n -, -C(=O)((CH 2) n O) t (CH 2) n X 1 (CH 2) n -, -C(=O)((CH 2) n O) t (CH 2) n NHC(=O)(CH 2) n -, -C(=O)((CH 2) n O) t (CH 2) n C(=O)NH(CH 2) n -, -C(=O)NH((CH 2) n O) t (CH 2) n X 1 (CH 2) n -, -C(=O)X 2 X 3 C(=O)((CH 2) n O) t (CH 2) n -,-C(=O)X 2 X 3 C(=O)(CH 2) n -, -C(=O)X 2 C(=O)(CH 2) n NHC(=O)(CH 2) n -,-C(=O)X 2 C(=O)(CH 2) n NHC(=O)((CH 2) n O) t (CH 2) n -, -C(=O)(CH 2) n C(R 7) 2 -,-C(=O)(CH 2) n C(R 7) 2 SS(CH 2) n NHC(=O)(CH 2) n -, -(CH 2) n X 2 C(=O)(CH 2) n NHC(=O)((CH 2) n O) t (CH 2) n - or -C(=O)(CH 2) n C(=O)NH(CH 2) n; R 40 is -S-,-NHC(=O)CH 2 -, -S(=O) 2 CH 2 CH 2 -, -(CH 2) 2 S(=O) 2 CH 2 CH 2 -, -NHS(=O) 2 CH 2 CH 2,-NHC(=O)CH 2 CH 2 -, -CH 2 NHCH 2 CH 2 -, -NHCH 2 CH 2 -, or X 1 is X 2 is X 3 is each R 7 is independently selected from H and C 1 -C 6 alkyl; each R 8 is independently selected from H, C 1 -C 6 alkyl, F, Cl, and -OH; each R 9 is independently selected from H, C 1 -C 6 alkyl, F, Cl, -NH 2, -OCH 3, -OCH 2 CH 3,-N(CH 3) 2, -CN, -NO 2 and -OH; each R 10 is independently selected from H, C 1-6 alkyl, fluoro, benzyloxy substituted with-C(=O)OH, benzyl substituted with -C(=O)OH, C 1-4 alkoxy substituted with -C(=O)OH and C 1-4 alkyl substituted with -C(=O)OH; R 12 is H, methyl or phenyl; each m is independently selected from 1, 2, 3, and 4; each n is independently selected from 1, 2, 3, and 4; each t is independently selected from 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 and 18, and y is an integer from 1 to 16.