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The H I 21 cm transition provides a valuable and unique probe of the roles that gas plays in galaxy evolution and cosmology, and our understanding of these roles is far from complete. This talk discussed the synergy obtained by combining 21 cm observations with ultraviolet measurements of QSO absorption lines. In some respects, QSO absorption spectroscopy provides a much more sensitive tool for the study of intergalactic and interstellar gases—QSO absorption is six orders of magnitude more sensitive to NHI than 21 cm emission, and absorption spectroscopy reveals a rich array of metals including low-, intermediate-, and high-ionization stages and even various molecules—but the UV absorption data only probes the pencil beam to the background QSO. This brief contribution provides an example of how the combination of UV absorption spectroscopy and 21 cm emission provides a more complete picture of the nature of the gas. [ABSTRACT FROM AUTHOR]