First Direct Measurement of P$_{\mu}\xi$ by TWIST

The TWIST experiment is completing its first direct measurement of the Muon Decay Parameter P$_{\mu}\xi$. Together with the other Michel parameters describing normal muon decay, $\xi$ provides sensitive tests of the Standard Model of electroweak interactions. This talk will discuss the measurement and the assessment of systematic uncertainties, which dominate the precision of the result. The physics implications will be presented.