In the recent advancement of piezoelectric resonator technology, there has been a large growth in the application of these devices for chemical sensing. These sensors operate by detecting changes in their environment which perturb the electrical - acoustic operation and in turn can be harnessed by means of supporting electronics and signal processing to monitor various processes. Examples include remote environmental monitoring, chemical process control, and commercial gas phase detectors. In this dissertation, the chemical sensing theory and properties of piezoelectric resonators such as the bulk-acoustic wave thin-film resonator (TFR) and the quartz crystal microbalance (QCM) are developed. This analysis concentrates on characterizing the resonance behavior of thickness mode resonators based upon the physical properties at the electrode interface which include interfacial mass density, elasticity, viscosity, and thickness of the composite device consisting of the piezoelectric material, the electrodes, and any deposited layer on the electrode surface in contact with the surrounding medium. In this work, no approximation is made as to the stress or particle displacement variation across the visco-elastic film which allows a complete study of the perturbational mechanical variations on the electrical and resonance properties of the composite resonator. The derivation and verification of equivalent circuit models based on the physical properties of the piezoelectric resonator and visco-elastic sensing film are presented. The results and models from this research will be beneficial to surface chemistry studies and also have application to fabrication techniques and electrical modeling. The use of this theory is employed in a study of a QCM coated with a commercially developed negative resist. Photo-polymerization of the resist results in induced visco-elastic structural changes which can be monitored and characterized using the full admittance theory of the composite thickness mode resonator. In order to validate the chemical sensing concept, the design and implementation of a TFR controlled chemical sensing system is demonstrated. This system employs the frequency selectivity of the chemical sensing TFR as the feedback element in integrated Colpitts oscillators which are downconverted by superheterodyne techniques. The integrated system design philosophy and performance tradeoffs are discussed. This analysis also investigates the phase noise performance and injection locking considerations of the design. The sensor system detection limit is derived which sets the lower limit of signal detection based upon measurand sensitivity and measured phase noise.