  Spectral distortions and anisotropies of the CMB provide independent and
complementary probes to study energy injection processes in the early universe.
Here we discuss the synergy between these observables, and show the promising
future of spectral distortion missions to constrain both exotic and non-exotic
energy injections. We show that conventional probes such as Big Bang
Nucleosynthesis and CMB anisotropies can benefit from and even be surpassed by
future spectral distortion experiments. For this, we have implemented a unified
framework within the Boltzmann code CLASS to consistently treat the thermal
evolution of photons and baryons. Furthermore, we give an extensive and
pedagogical introduction into the topic of spectral distortions and energy
injections throughout the thermal history of the universe, highlighting some of
their unique features and potential as a novel probe for cosmology and particle
physics.
