This thesis is a study of inverse problems in fractional calculus where two major theorems of existence and uniqueness for the solution of fractional order integro differential equations have been proved in different approaches. Delay fractional order integro differential equations have also been discussed; some related results have been achieved. Also, three methods for solving inverse problems of fractional order integro differential equations with and without delay have been improved. These methods were illustrated by some examples. The computer implementations needed in this work have been written by Quick Basic programming language. Some applications of problems associated with fractional order integro differential equations have been presented and solved by the improved methods. Finally, the results of the work have been discussed, and some recommended proposals and for future work have been suggested.