An attempt is made to define the concept of execution of an instruction sequence. It is found to be a special case of directly putting into effect of an instruction sequence. Directly putting into effect of an instruction sequences comprises interpretation as well as execution. Directly putting into effect is a special case of putting into effect with other special cases classified as indirectly putting into effect. Indirectly putting into effect an instruction sequence refers to a class of mechanical processes by default, which includes mechanisms making use of preparatory transformations and compilations of the entire instruction sequence or of significant parts thereof, where the transformations and compilations are themselves produced by means of putting yet other instruction sequences into effect. Returning to instruction sequence execution, its initial informal definition is: directly putting into effect an instruction sequence in such a way that it does not represent a form of interpretation, where a method of directly putting into effect is considered an interpretation if it represents an instance of the functionality produced by putting some other instruction sequence into effect (either directly or indirectly).