  We discuss a new class of quantum phase transitions -- Deconfined Mott
Transition (DMT) -- that describe a continuous transition between a Fermi
liquid metal with a generic electronic Fermi surface and an electrical
insulator without Fermi surfaces of emergent neutral excitations. We construct
a unified $U(2)$ gauge theory to describe a variety of metallic and insulating
phases, which include Fermi liquids, fractionalized Fermi liquids (FL*),
conventional insulators and quantum spin liquids, as well as the quantum phase
transitions between them. Using the DMT as a basic building block, we propose a
distinct quantum phase transition -- Deconfined Metal-Metal Transition
(DM$^2$T) -- that describes a continuous transition between two metallic
phases, accompanied by a jump in the size of their electronic Fermi surfaces
(also dubbed a 'Fermi transition'). We study these new classes of deconfined
metallic quantum critical points using a renormalization group framework at the
leading nontrivial order in a controlled expansion, and comment on the various
interesting scenarios that can emerge going beyond this leading order
calculation. We also study a $U(1)\times U(1)$ gauge theory that shares a
number of similarities with the $U(2)$ gauge theory and sheds important light
on many phenomena related to DMT, DM$^2$T and quantum spin liquids.
