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Extended abstractThe mass-fraction Y of helium in the interstellar medium is between 0.22 and 0.30 wherever it has been measured and it is believed to be the sum of two components: YP from Big Bang nucleosynthesis (BBNS) at about 100 s after the Big Bang (ABB) and a temperature near 0.1 MeV, and ΔY due to processing in stars. Precise measurements of Yp, along with balances of trace elements D, 3He, 7Li also resulting from BBNS, provide important tests of BBNS theory and of parameters of cosmology and particle physics, notably the contribution ΩBO of baryons to the mean density of matter in the universe (in units of the closure density), the number Nv of light neutrino flavours (or families of quarks and leptons) and the half-life т½ of the neutron (Shaver et al. 1983; Yang et al. 1984; Boesgaard and Steigman 1985). Figure 1 shows the predicted abundances from Standard BBNS theory (SBBN) as a function of η = μB/nλ the ratio of baryons to photons (unchanged since e± annihilation a few seconds ABB), which is proportional (through the known temperature of the microwave background) to ΩBOh20 where h0 is the Hubble constant in units of 100 km s−1 Mpc−1. SBBN theory (which assumes a homogeneous Friedmann universe and small lepton numbers), when combined with reasonable ideas on Galactic chemical evolution that predict a primordial (D + 3He)/H ratio below 10−4, imply that η ≥ 3 × 10−10 (shown by the tall vertical line in Fig. 1), which in turn implies YP≥0.210 if Nv = 3 and т½≥10.4 minutes. But this limit can be somewhat relaxed if т½ is smaller (current measurements permit values down to 9.0 minutes, e.g. Last et al. 1988) and/or if the quark-hadron phase transition around 200 MeV is first-order and leads to significant density fluctuations (Kurki-Sunonio et al. 1989; Reeves 1989).