With the parameters of the standard cosmological model constrained by obervations of the cosmic microwave background, the Hubble constant is about 3.5 standard deviations lower than that determined from traditional distance ladder methods as reported by Riess et al. (2016). In this talk I will explain how CMB data lead to constraints on the Hubble constant, and present the case that, given the assumption of the standard cosmological model, the CMB predictions for the Hubble constant are quite robust. I will also present our re-analysis of the Cepheid data used by Riess et al. (2016), a re-analysis that supports their conclusions. Finally, I’ll discuss potential changes to the cosmological model that could help to resolve the discrepancy.