BBN constraints on the universal density of baryons and on the early-Universe expansion rate require reasonably accurate determinations of the relic abundances of the light nuclides. As already noted, D, 3He, and 7Li are all potential baryometers, while 4He is an excellent chronometer. The combination of the availablility of large telescopes and advances in detector technology has made it possible to obtain abundance estimates at various sites in the Galaxy and elsewhere in the Universe with unprecedented precision (statistically). However, the path to accurate primordial abundances is littered with systematic uncertainties which have the potential to contaminate otherwise exquisite data. It is, therefore, fortunate that the relic nuclides follow very different post-BBN evolutionary paths and are observed in diverse environments using a wide variety of astronomical techniques. Neutral deuterium is observed in absorption in the UV (or, in the optical when redshifted) against background, bright sources (O or B stars in the Galaxy, QSOs extragalactically). Singly-ionized helium-3 is observed in emission in Galactic HII regions via its spin-flip transition (the analog of the 21 cm line in neutral hydrogen). The helium-4 abundance is largely determined by observations of recombination lines of ionized (singly and doubly) 4He compared to those of ionized hydrogen in Galactic and, especially, extragalactic HII regions. Observations of 7Li, at least those at low metallicity (nearly primordial) are restricted to absorption in the atmospheres of the oldest, most metal-poor stars in the halo of the Galaxy. The different evolutionary histories (described below) combined with the differrent observational strategies provide a measure of insurance that systematic errors in the determination of one of the light element abundances are unlikely to propagate into other abundance determinations.