The manner in which states of some quantum systems become effectively classical is of great
significance for the foundations of quantum physics, as well as for problems of practical interest such
as quantum engineering. In the past two decades it has become increasingly clear that many (perhaps
all) of the symptoms of classicality can be induced in quantum systems by their environments. Thus
decoherence is caused by the interaction in which the environment in effect monitors certain
observables of the system, destroying coherence between the pointer states corresponding to their
eigenvalues. This leads to environment-induced superselection or einselection, a quantum process
associated with selective loss of information. Einselected pointer states are stable. They can retain
correlations with the rest of the universe in spite of the environment. Einselection enforces classicality
by imposing an effective ban on the vast majority of the Hilbert space, eliminating especially the
flagrantly nonlocal "Schrodinger-cat states." The classical structure of phase space emerges from the
quantum Hilbert space in the appropriate macroscopic limit. Combination of einselection with
dynamics leads to the idealizations of a point and of a classical trajectory. In measurements,
einselection replaces quantum entanglement between the apparatus and the measured system with the
classical correlation. Only the preferred pointer observable of the apparatus can store information
that has predictive power. When the measured quantum system is microscopic and isolated, this
restriction on the predictive utility of its correlations with the macroscopic apparatus results in the
effective "collapse of the wave packet." The existential interpretation implied by einselection regards
observers as open quantum systems, distinguished only by their ability to acquire, store, and process
information. Spreading of the correlations with the effectively classical pointer states throughout the
environment allows one to understand "classical reality" as a property based on the relatively
objective existence of the einselected states. Effectively classical pointer states can be "found out"
without being re-prepared, e.g, by intercepting the information already present in the environment.
The redundancy of the records of pointer states in the environment (which can be thought of as their
"fitness" in the Darwinian sense) is a measure of their classicality. A new symmetry appears in this
setting. Environment-assisted invariance or envariance sheds new light on the nature of ignorance of
the state of the system due to quantum correlations with the environment and leads to Born's rules
and to reduced density matrices, ultimately justifying basic principles of the program of decoherence
and einselection.