  We derive new consistency conditions for string compactifications with
generic fluxes (RR, NSNS, geometrical) and localized sources (D-branes,
NS-branes, KK-monopoles). The constraints are all related by string dualities
and share a common origin in M-theory. We also find new sources of
instabilities. We discuss the importance of these conditions for the
consistency of the effective action and for the study of interpolating
solutions between vacua.
