  The cosmic microwave background (CMB) offers a unique window into the early
universe, providing insights into cosmological parameters like the Hubble
constant. Recent precise measurements of the CMB by experiments like Planck
seem to point to a lower value for the Hubble constant compared to some other
measurements like those from Type Ia supernovae. This discrepancy, known as the
Hubble tension, currently lacks a definitive explanation. In this chapter, we
provide an overview of how the Hubble constant is determined from detailed
measurements of the CMB power spectrum. We explain the physics underlying key
features of the CMB spectrum and their connection to cosmological parameters.
We then examine the consistency of Planck's Hubble constant determination, both
internally within the data itself and externally with other astrophysical
probes. While largely consistent, some anomalies like the lensing amplitude
parameter $A_L$ remain unresolved. We also explore various theoretical
extensions to the standard ${\Lambda}$CDM cosmological model and assess their
potential to resolve the Hubble tension. No clear resolution emerges,
indicating significant tensions remain between early and late universe probes
within simple extensions to ${\Lambda}$CDM. Upcoming CMB experiments promise
improved precision and should provide further insights into this cosmic
conundrum. A coherent picture bridging measurements across cosmic time remains
an open challenge at the forefront of modern cosmology.
