  Thermodynamic properties and structural aspects of the non-stoichiometric
w\"ustite Fe 1-z O, and its modifications-the so-called pseudo-phases-as
functions of composition z and equilibrium temperature are reviewed from 1960
to present (159 references). The focus is first put on the complexity of the
equilibrium phase diagram. The first order transition W $\rightleftharpoons$ W'
is specified on the boundary iron/ w\"ustite close to 1185 K. Transitions
correlated to the modifications Wi at T(W) > 1185 K and W'j at 1185K < T(W') <
T(W) (i and j =1,2,3) are reexamined. Structural determinations based on point
defects and their clustering are recalled. A series of equilibria are
developed, trying to justify the stabilization of the pseudo-phases, which can
be interpreted in terms of transformation of defect clusters, or their mode of
distribution (percolation, superstructure) including changes in electronic
charge carriers.
