  Disorder on the active d element site is usually very disruptive for
conduction and long range order in perovskite transition metal oxides. However,
in the background of phase competition such `B site' dopants also act to
promote one ordered phase at the expense of another. This occurs either through
valence change of the transition metal or via creation of `defects' in the
parent magnetic state. We provide a framework for understanding the complex
variety of phenomena observed in B site doped manganites and identify the key
parameters that control the physics. Using a spatially resolved analysis of B
ions in various manganite phases we explain the existing data and predict new
situations where highly polarisable phase separated states can be created.
