Prof.~Thomas Mehen works primarily on Quantum Chromodynamics (QCD) and the application of effective field theory (EFT) to problems in hadronic physics. EFTs rely on three key ideas: i) identifying the relevant degrees of freedom for a specific physical process; ii) using symmetries of QCD to simplify or constrain the form of interactions; and iii) finding small parameters, either small coupling constants or ratios of disparate mass scales, which can be used to formulate systematic perturbative calculations of observables.