Functoriality is one of the most central questions in the theory of automorphic forms and representations [1,2,35,36]. Locally and globally, it is a manifestation of Langlands' formulation of a non-abelian class field theory. Now known as the Langlands correspondence, this formulation of class field theory can be viewed as giving an arithmetic parameterization of local or automorphic representations in terms of admissible homomorphisms of (an appropriate analogue) of the WeilDeligne group into the Langlands dual group or L-group. When this conjectural parameterization is combined with natural homomorphisms of the L-groups it predicts a transfer or lifting of local or automorphic representations of two reductive algebraic groups. As a purely automorphic expression of a global non-abelian class field theory, global functoriality is inherently an arithmetic process.In this paper we establish global functoriality from the split classical groups G n = SO 2n+1 , SO 2n , or Sp 2n to an appropriate general linear group GL N , associated to the natural embedding of L-groups, for globally generic cuspidal representations π of G n (A) over a number field k. We had previously presented functoriality for the case G n = SO 2n+1 in [6], but were limited at that time by a lack of suitable local tools in the other cases. The present paper is by no means a simple generalization of [6]. There were serious local problems to be overcome in the development of the tools that now allow us to cover all three series of classical groups simultaneously and that will be applicable to other cases of functoriality in the future. In addition, we have completely determined the associated local images of functoriality and as a result are able to present several new applications of functoriality, including both global results concerning the Ramanujan conjecture for the classical groups and various applications to the local representation theory of the classical groups.There are several approaches to the question of functoriality: the trace formula, the relative trace formula, and the Converse Theorem. In this work we use the Converse Theorem, which is an L-function method. The Converse Theorem itself states that if one has an irreducible admissible representation Π ⊗ Π v of GL N (A), then Π is in fact automorphic if sufficiently many of its twisted L-functions L(s, Π × τ), with τ cuspidal automorphic representations of smaller GL m (A), are nice [7,9]. As a vehicle for establishing functoriality from cuspidal representations π = ⊗π v of some G n (A) to an automorphic representation of GL N (A), there are three main steps. The first is to construct a candidate lift Π = ⊗ Π v . This is done by locally lifting each local component representationin such a way that twisted local L-and ε-factors are matched. At the archimedean places and the finite places where π v is unramified we may accomplish this local lift by using the local Langlands correspondence, i.e., the local arithmetic Langlands classification. At the remaining finite set of places where π...