CMB measurements of $H_0$

The cosmic microwave background (CMB) encodes information about the expansion history of the universe both before the radiation decoupled from the matter, and since that decoupling. Deducing the current value of the Hubble parameter from the CMB data amounts to a stringent test of the standard cosmological model. All measurements of $H_0$ require excellent understanding of, and strict control of, systematic effects on the data. The best constraints on $H_0$ from the CMB rely on the Planck data. I will describe an upcoming complementary path to estimating $H_0$ from a combination of ACT and WMAP data, with similar constraining power to the Planck results.