  In this paper, the ${\cal A}$-theory, an extension of F-theory, is described
as a fully U-duality covariant brane theory. This theory has some
distinguishing features not known from world-sheet models. In particular, seen
as a sigma model, both world-volume and target space coordinates are specific
representations of the same group (the U-duality group). The U-duality group in
question is an exceptional group (a split form of the $E_d$ series). The
structure of this group allows it to encompass both the T-duality group of
string theory as well as the general linear symmetry group of ${\cal
M}$-theory. ${\cal A}$-theory is defined by the current algebras in Hamiltonian
formalism, or by world-volume actions in Lagrangian formalism. The spacetime
coordinates are selfdual gauge fields on the world-volume, requiring the Gauss
law constraints tying the world-volume to spacetime. Solving the Gauss law
constraints/the Virasoro constraints gives the world-volume/spacetime
sectioning from ${\cal A}$-theory to ${\cal T}$-theory/ ${\cal M}$-theory
respectively. The ${\cal A}$-theory Lagrangian admits extended symmetry which
has not been observed previously in the literature, where the background fields
include both the spacetime and the world-volume gravitational fields. We also
constructed the four-point amplitude of ${\cal A}$-theory in the low energy
limit. The amplitude is written in a way that the U-duality symmetry is
manifest, but after solving the section condition, it reduces to the usual
four-graviton amplitude. In the previous papers, we have referred to this model
as F-theory, however, F-theory initiated by Vafa is now a big branch of string
theory as the study of elliptic fibrations, so we refer to these constructions
as generalized models of theory for all dimensions with all duality symmetries
as ${\cal A}$-theory.
