The effect of disorder due to nomagnetic impurities on superconductivity of Bi2Sr2CaCu2O8+xrnwas investegated theoretically employing the matrix Green function in Nambu formalism.rnThe relationship between disorder parameter and superconducting critical temperaturernhas been calculated analytically for each of its possible pairing symmetries. The obtainedrnresults indicate that diluted nonmagnetic disorder has no significant e_ect on superconductingrncritical temperature of the system when the pairing symmetry is s-wave. On thernother hand, the results obtained for the d-wave pairing symmetries show that disorder canrnsignificantly suppress the superconducting critical temperature of the system. Moreover,rnthe results reveal that the supression of Tc depends on the strength of the impurityrnpotential, the concentration of scatterers and the anisotropy of the pairing symmetries.rnThe e_ect of disorder due to randomness in island potential on superconductivityrnof thin films was also investigated theoretically based on the hard core boson modelrndeveloped for superfluid helium. The relationship between superconducting orderrnparameter and disorder paramters was found analytically using the double time retardedrnGreen function formalism. The obtained results indicate that disorder induced due tornrandomness in island potential enhances decoherence of Cooper pairs and suppressesrnsuperconducting order parameter and superfluidity of the system. The results alsornindicate that increasing the two-body interaction strength suppresses superconductivityrnof the system. The critical values of disorder and interactin strength at which the systemrncompletely ceases its superconductivity was also determined. Below these critical values,rnthe system acts as a superconductor, a system with zero electrical resistance. Above therncritical values, it acts as an insulator, a system with infinite electrical resistance.rnFinally, the e_ect of pressure on superconductivity of Pb and MgB2 was investigatedrntheoretically based on density functional theory as implemented in Quantum ESPRESSO