Data on the luminosity function (LF) of galaxies are reviewed and compared on a common magnitude system and with common assumptions. The result of Kirshner et al. (Astron. J. 88, 1285, 1983) is chosen as a best guess. Departures from this “standard LF” for specific galaxy types and environments (clusters, groups, field) are discussed briefly. If the Galactic absorption is AB = 0.2 csc|b| and the solar absolute magnitude is MB⊙ = 5.48, this LF leads to a mean luminosity density on the BT system, or about 1.4 × 10−2 h “Galaxies” Mpc−3. The mean M/L ratio needed to give critical cosmological density (Ω0 = 1) is then 920h in solar units on the BT0 (face-on) system. [The latter number can be larger by a factor ∼2 if a different LF is used, and larger still on different systems and with different assumptions (see Table).] The “weighed” M/L on this system is in clusters, but it is smaller in binaries and small groups. Estimates of the weighed (clumped) Ω0 vary, but it is definitely < 1. Comparison with constraints imposed by inflation and nucleosynthesis suggests that we distinguish at least two “dark-matter” problems: (1) What is the weighed mass, and how is it distributed? It contributes an Ω0 ∼ 0.1–0.5 and could be all baryonic if at the lower end of this range. (2) Is there additional matter, more smoothly distributed and probably nonbaryonic, which brings Ω0 up to unity? For details see Comments on Astrophys., in press.