The basic electroweak tlimry of atomic parity non-conservation (PNC) is reexamined. A formalism is used in which the result is largely independent of the weak mixing angle and can be directly expressed in term of accurately known quantities. While the main thrust concerns PNC in a single isotope, isotopic diRerences are also covered for completeness. Electroweak radiatiw corrections are re-calculated at the I-loop level. Considerable emphasis is then placed on certain 2-loop improvements which enler a1 aboul the 0.015 level. The overall result agrees with the work of Marciano and Rosner. Contributions from SU(2), x 4 1 ) heavy physics beyond the standard model are included and a value S= -2.8f2.1,,,+ I.2,k., is found. The implications of this result are reviewed * Q~= p ' " ' = 1,
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