The ultrasonics are becoming increasingly important waves in the electrical engineering technologies including medical electronics. Among key technologies in the field of applications of ultrasonics, piezoelectric materials are the most fundamental. In the present review, 10 items of fundamentals on the piezoelectric materials are described in a question-and-answer form. A short history of piezoelectric materials, an overview of basic properties of piezoelectric crystals and ceramics, and derivations and interpretations of the fundamental equations on electromechanical conversions are mainly described. Some numerical calculations of piezoelectric effects on piezoelectric materials are also presented.