The results of substitution of A (with A = Ni and Zn) for Cu are presented in the high Tc superconductors, YBa₂(Cu_{1 - x}A_x)₃O_{6 + δ} where x < 0.03 for Zn and x< 0.10 for Ni. We have observed that the superconducting temperature in the high Tc YBa₂Cu₃O_{6 + δ} as system decreases if copper is substituted by Ni and Zn and the substitution of Zn affects Tc much more than Ni. We discuss our results in terms of pair breaking by magnetic impurities and in terms of the Resonance Valence Bond model proposed by Anderson. / Master of Science