Bachiller et al. 1997). Millimeter CO emission has proved to be the most generally useful probe of the warm molecular component of the neutral gas for the same reasons that it is a standard probe of the precursor AGB envelopes: CO is relatively abundant and the millimeter transitions are easily excited. It can therefore be used to study the mass, structure, and kinematics of the bulk of the molecular gas.

Most of the known CO envelopes around PNe were discovered in dedicated CO surveys carried out by Huggins & Healy (1989) using the NRAO 12 m telescope, and by Huggins et al. (1996) using the SEST 15 m and IRAM 30 m telescopes. The compilation in Table 3 of Huggins et al. (1996) lists 44 PNe detected in CO. Several of these have provided examples for detailed study at high angular resolution, and interpretation of the ensemble of the CO observations forms an integral part of the evidence for the evolutionary scenario outlined above.

Several recent lines of investigation, including optical imaging of PNe (e.g., Sahai & Trauger 1998; Lopez 2003), kinematic studies of proto-PNe (e.g., Bujarrabal et al. 2001), and observations of individual molecular envelopes (e.g., He 3-1475, Huggins 