Molecular gas is arguably one of the most important constituents to consider in any study of galaxy evolution. It is the raw material from which stars form, and it responds strongly to dynamical influences which can move large amounts of gas. Spiral galaxies, in particular, provide a variety of environments in which to study the processing of molecular gas. For instance, galactic bars are likely important in fueling central star formation by promoting the flow of gas inward. The star formation resulting from the influx of gas may in turn promote the development of a bulge-like structure, or even the destruction of the bar itself (e.g., , , ). In addition, the presence of a bar or spiral potential may influence the large-scale pattern of gas, and therefore new star formation (e.g., , , ).
The symmetry of the H2 molecule effectively limits the direct detection of molecular material to the small fraction of warm molecular gas typically found in the vicinity of shocks or star formation. Due to its relatively high abundance and low effective excitation, the CO molecule has become a standard proxy for H2, and its emission (particularly from the J = 1-0 transition) has been detected in many galaxies, in some cases out to very large redshift (e.g., z = 6.419, ). There is still significant debate about the appropriate conversion factor between CO integrated intensities and H2 surface densities in different environments (see presentations by M. Guelin, F. Walter, and A. Bolatto in this volume; see also, e.g., , , , , ). However, there is no question that CO observations play a key role in discovering the whereabouts and motions of molecular gas in galaxies outside the Milky Way.
In the last decade, the improved sensitivity of millimeter-wave facilities has enabled studies of the distribution and kinematics of molecular gas in galaxies on smaller spatial scales, at larger radii, and over smaller velocity shifts. In particular, the advent of a number of surveys has significantly advanced the study of molecular gas in nearby galaxies since the previous Zermatt Symposium. We now have a full accounting of the molecular gas in Local Group spirals, as well as a self-consistent accounting of the detailed molecular gas distributions in wider samples of Hubble type, bar contribution, and star formation activity. For the purposes of this presentation, I include techniques such as On The Fly (OTF) mapping (with single-dish telescopes) and mosaicking (for interferometers) as "survey" techniques.