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The effect of silver (Ag) addition on the resistance to thermal shock and delayed failure of YBa2Cu3O7_s superconducting wires has been evaluated. Resistance to thermal shock was evaluated by measuring the critical-current density (Jc) as a function of the number of thermal cycles for YBCO and its composites with 15 vol. % Ag additions. Composite YBCO-Ag wires show a slower decrease in Jc than does monolithic YBCO as the number of thermal cycles increases. The nature of delayed failure was studied by measuring strength (erf) as a function of loading rate (&) and evaluating the value of the subcritical crack growth parameter N[