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The part-per-million measurement of the positive muon lifetime anddetermination of the Fermi constant by the MuLan experiment at the PaulScherrer Institute is reviewed. The experiment used an innovative,time-structured, surface muon beam and anear-4 \pi π ,finely-segmented, plastic scintillator positron detector. Two in-vacuummuon stopping targets were used: a ferromagnetic foil with a largeinternal magnetic field, and a quartz crystal in a moderate externalmagnetic field. The experiment acquired a dataset of 1.6 \times 10^{12} 1.6 × 10 12 positive muon decays and obtained a muon lifetime \tau_{\mu} = 2\, 196\, 980.3(2.2) τ μ = 2 196 980.3 ( 2.2 ) ~ps(1.0~ppm) and Fermi constant G _F = 1.166\, 378\, 7(6) \times 10^{-5} F = 1.166 378 7 ( 6 ) × 10 − 5 GeV ^{-2} − 2 (0.5~ppm). The thirty-fold improvement in \tau_{\mu} τ μ has proven valuable for precision measurements in nuclear muon captureand the commensurate improvement in G _F F has proven valuable for precision tests of the standard model.