A modular holder or patch is described that may be used with or as part of a wireless physiological sensing device. The wireless physiological sensing device may include a holder or patch, first and second electrodes, and an electronics package that may be removably coupled with the holder or patch and which may be in electrical contact with the first and second electrodes. The electronics package may include a housing, a wireless transceiver and electronic circuitry configured to process signals received via the first and second electrodes and the wireless transceiver.