Muon decay is an excellent place
to study the structure of the weak interaction. Since it is a purely
leptonic process, many uncertainties deriving from the internal
strucure of the particles involved or from other interactions are
eliminated. TWIST measures
with high precision the momentum and angular dependence of positrons
emitted from the decay of polarized positive muons. I will describe the
apparatus and some of the analysis techniques that we have employed.
New results for the muon decay parameters rho and delta will be
presented, together with their implications regarding the spacetime
symmetries of the weak interaction.