I will review the constraints on H_0 from the Planck CMB measurements and compare the best fit Planck model with recent results from ACTpol.
The CMB value of H_0 is secure, yet is in tension with Cepheid based measurements of H_0. Although the CMB value is based on theLambda CDM cosmology, there has been no compelling theoretical modification of the model that can explain the tension. I will therefore explore whether systematic errors may be affecting the Cepheid distance scale and if so, what must these errors look like.