  We review various K-theory classification conjectures in string theory. Sen
conjecture based proposals classify D-brane trajectories in backgrounds with no
H flux, while Freed-Witten anomaly based proposals classify conserved RR
charges and magnetic RR fluxes in topologically time-independent backgrounds.
In exactly solvable CFTs a classification of well-defined boundary states
implies that there are branes representing every twisted K-theory class. Some
of these proposals fail to respect the self-duality of the RR fields in the
democratic formulation of type II supergravity and none respect S-duality in
type IIB string theory. We discuss two applications. The twisted K-theory
classification has led to a conjecture for the topology of the T-dual of any
configuration. In the Klebanov-Strassler geometry twisted K-theory classifies
universality classes of baryonic vacua.
