Piezoelectric crystals play an important role in our day to day technologically advanced world. In this book the elastic properties of two distinct groups of piezoelectric crystals are presented: one with small piezoelectric properties for eg. natural tourmaline; while on the other hand with huge piezoelectric constants i.e. Czochralski grown pure and cerium doped relaxor ferroelectric calcium barium niobate (Ca0.28Ba0.72Nb2O6 hereafter CBN-28) exhibiting a ferroelectric-paraelectric phase transition. To this end a non-destructive innovative method of resonant ultrasound spectroscopy (RUS) was employed. Evidence of the existance of Burns temperature in CBN-28 type relaxor has been reported for the first time.