  The synthesis of transition metal heterostructures is currently one of the
most vivid fields in the design of novel functional materials. In this paper we
propose a simple scheme to predict \emph{band alignment }and \emph{charge
transfer} in complex oxide interfaces. For semiconductor heterostructures band
alignment rules like the well known Anderson or Schottky-Mott rule are based on
comparison of the work function or electron affinity of the bulk components.
This scheme breaks down for oxides due to the invalidity of a single
workfunction approximation as recently shown (Phys. Rev. B 93, 235116; Adv.
Funct. Mater. 26, 5471). Here we propose a new scheme which is built on a
continuity condition of valence states originating in the compounds' shared
network of oxygen. It allows for the prediction of sign and relative amplitude
of the intrinsic charge transfer, taking as input only information about the
bulk properties of the components. We support our claims by numerical density
functional theory simulations as well as (where available) experimental
evidence. Specific applications include i) controlled doping of SrTiO$_3$
layers with the use of 4$d$ and 5$d$ transition metal oxides and ii) the
control of magnetic ordering in manganites through tuned charge transfer.
