We review observations to date of silicon-bearing molecules in circumstellar gas and dust around red giants. New constraints on circumstellar chemistry are provided by the discovery of (i) a systematic decrease in the [SiO]/[H2] ratio with mass loss rate, ii) SiO in carbon-rich CIT 6. The presence/absence of SiO maser emission in oxygen-rich/carbon-rich envelopes is linked to differences in grain composion and the grain-condensation radius.