A brief survey of ferroelectrics and a statement of the purpose of the present research are given. The necessary piezoelectric equations are developed and, using Mueller's theory of Rochelle salt as a basis, a phenomenological theory for potassium dihydrogen phosphate is worked out. Measurements of the electro-mechanical constants in the Curie region are reported. The experimental results are discussed in terms of the phenomenological theory and compared with a theory proposed by W.P.Mason. Since neither offers on adequate explanation of the results, the possibility of domain wall movement is considered. Crystal growth and experimental techniques are also discussed.