Broad emission lines provide the most readily available abundance diagnostics because they are present in all QSOs. Many BELs can be measured in large QSO samples using medium-resolution spectra. However, the strengths of the metal lines, such as CIV 1549, relative to Ly are surprisingly insensitive to the global metallicity (HF99, Hamann & Ferland 1993b). QSOs could actually have a very wide range of metallicities while displaying qualitatively similar BEL spectra (also Korista et al. 1998). Nonetheless, some line ratios are sensitive to the relative abundances. The challenge is to identify observable lines that have significant abundance sensitivities above their unavoidable dependences on other factors. In general, we must rely computational models to quantify the various parameter sensitivities. Since the BEL region (BELR) spans a range of radii (Peterson 1993) and probably has a wide range of densities and ionizations (Ferland et al. 1992, Baldwin et al. 1995, Hamann et al. 1998), the most important criterion for abundance-sensitive line ratios is that they form under similar conditions (e.g. in the same or overlapping regions). Line ratios involving nitrogen should be particularly useful for constraining the total metallicity and evolutionary state of QSO environments (Section 2.4). Boyle (1990), Laor et al. (1995) and Osmer & Shields (this volume) present labeled plots showing the UV emission lines that are generally available for abundance studies.