Neutrinos and new light particles, which arise in many extensions of the Standard Model of particle physics, are an important probe of the (thermal) history of the universe. However, they are hard to detect in terrestrial experiments due to their weak couplings to ordinary matter. On the other hand, the high temperatures in the early universe allow an efficient production of even very weakly coupled particles. I will discuss how measurements of the cosmic microwave background and the large-scale structure of the universe, e.g. by Planck, CMB-S4 and DESI, can shed new light on the possible existence of light scalar particles beyond the Standard Model, such as axions. I will in particular present bounds on their interactions with Standard Model fermions that can be inferred with current data and will improve considerably with future experiments, in particular CMB-S4. In addition, I will compare these cosmological constraints to astrophysical bounds and highlight their complementarity, in particular for couplings to muons and SN 1987A.