What happens to talented musicians during the actual performance of their musical numbers that results in performances that are less than their best? Why do two musicians of comparable abilities give performances of such divergent qualities? What can a music teacher do to help or encourage the underproductive musician? To answer these questions some music educators have turned to the psychologist Psychologists are making breakthroughs in the explorations of personalities in conflict with self and their environment. In this paper some applications of psychological principles to musical development and performance skills are made in the area of anxiety and its relief. The application of the critical periods in language development to development in music as a special case of language is also made in this thesis.