In this talk, I'll delve into recent work centred on muons, highlighting developments in two key areas: the Mu3e experiment's search for light new particles and the study of muon behaviour in neutron stars. I'll discuss how the Mu3e experiment is uniquely positioned to search for light new physics through resolving colinear electrons and positirons. For promptly decaying new resonances this allows Mu3e to probe a difficult corner of parameter space for dark photons and axion-like particles. While for long-lived new resonances I'll show how a new search strategy, requiring an additional e+ e- pair from internal conversion, can be used to circumvent the calibration challenges which plague the μ → e+X channel for X ≲ 20~MeV. At the end turning to astrophysics, I'll talk about the intriguing role of muons in neutron stars, where their interaction dynamics offer insights into the neutron stars' internal processes potentially leading to a non-thermal MeV-scale flux of neutrinos.