Recent experimental demonstrations of quantum coherence of the charge and flux states of Josephson junctions show that the quantum Josephson dynamics can be used to develop scalable quantum logic circuits. In this work, I review the basic concepts of Josephson tunneling and Josephson-junction qubits, and discuss two problems of this tunneling motivated by quantum computing applications. One is the theory of photon-assisted resonant flux tunneling in SQUID systems used to demonstrate quantum

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... rence of flux. Another is the problem of quantum measurement of charge with the SET electrometer. It is shown that the SET electrometer at the Coulomb blockade threshold is the quantum-limited detector with energy sensitivity reachingh/ √ 3 in the resonant-tunneling regime.