In this paper we introduce a class of mathematical objects called \emph{extensors} and develop some aspects of their theory with considerable detail. We give special names to several particular but important cases of extensors. The \emph{extension,} \emph{adjoint} and \emph{generalization} operators are introduced and their properties studied. For the so-called $(1,1)$-extensors we define the concept of \emph{determinant}, and their properties are investigated. Some preliminary applications of the theory of extensors are presented in order to show the power of the new concept in action. An useful formula for the inversion of $(1,1)$-extensors is obtained.