Abstract: The present invention is directed to anti-Her2 antibodies and fragments thereof, compositions, and uses thereof. The antibodies and fragments thereof bind to Her2-expressing cells in a pH-dependent manner, with low affinity at the physiological pH typical of normal cells and tissues, and with high affinity at the slightly acidic pH characteristic to solid tumors. Furthermore, the antibodies inhibit the growth of tumor spheroids at a level comparable to that of the benchmark anti-Her2 antibody Herceptin at acidic pH, whereas these effects were significantly reduced at physiological pH. This pH selectivity of cellular growth inhibition is maintained upon antibody conjugation to cytotoxic drugs. The invention also includes methods of treating solid tumors, methods of detecting solid tumors, and methods of pH-dependent capturing of the Her-2 ectodomain.