From automobile to aeronautic with biomedical, all applications domain uses sensors. Some of them are frequency-output. The coming of new piezoelectric materials opens a door to applications not accessible nowadays and specially at high temperatures. Gallium orthophosphate (GaPO4) is one of these new piezoelectric materials with high coupling factor. This book is about design and realisation of different GaPO's sensors working in acoustic bulk waves. The first part consists to study temperature sensibility and stability building analytical model of vibrating beam in length extensional, flexural and torsional modes.This theoretical study is then completed and compared with analysis using Finite Element Method (FEM) and experiemental measurements of some resonators made in different crystallographic orientations. Then application of biosensors using thickness shear mode is presented. An FEM is realised in order to study influence of some parameters on sensor behaviour. A comparison between model and measurements is also reported. These positive results open up possibilities of future developments described in conclusion.