Nonequilibrium quasiparticle excitations (QPs) are known to be a significant source of dissipation in superconducting quantum devices, though their observed density at 20 mK remains a mystery. In this talk, I will discuss our recent work utilizing offset-charge-sensitive transmons to probe QP dynamics. We detect QP tunneling events by coherent mapping of charge parity onto the qubit state and also by direct dispersive readout of the joint qubit and charge-parity state. We find that QP-induced dissipation can be on equal footing with other loss mechanisms and that QPs can be the dominant source of residual qubit excited-state population. In these devices, we also demonstrated that improved high-frequency rf filtering can reduce QP generation, extending a transmon’s relaxation time by a factor of two to ~200 microseconds.