We describe our measurements on high-precision microresonators and microresonator-based optical frequency combs, including full stabilization of planar frequency microcombs, with an instrument-limited instability at 3.6-mHz-per-root-tau and tooth-to-tooth relative uncertainty to 50mHz. Noise in thermally-compensated ultrahigh-Q microresonators and lasers are demonstrated at the thermodynamical limits.