A hallmark of quantum mechanics is the coherent superposition of states of a system, but uncontrolled interactions between system and environment lead to loss of coherence. For applications such as quantum computation and quantum metrology coherence must be preserved, and to obtain long coherence times a promising approach is to capitalize on the quantum properties of superconductors. Indeed many superconducting devices based on Josephson junctions are under active experimental investigation. This project has two main goals: to advance our theoretical understanding of decoherence processes in such superconducting devices and to explore how to limit their detrimental effects. The initial focus will be on the intrinsic decoherence due to quasiparticles, the elementary excitations in superconductors, especially under non-equilibrium conditions. The peculiarities of single, few, and many junction systems (such as transmon, phase and flux qubits, Cooper pair pumps, fluxonium, etc.) will be addressed in detail. Then the effectiveness of quasiparticle trapping schemes will be studied. In addition, interactions with photons will also be considered: on one hand, photons are used to manipulate these systems, on the other they cause decoherence, and the optimal balance between these two conflicting aspects will be sought. By suggesting new ways to reduce decoherence, the results of this project can contribute to the improvements in the performance of superconducting devices that will enable their practical use for quantum computation and metrology.