This is the first attempt of a systematic study of real Enriques
surfaces culminating in their classification up to deformation. Simple
explicit topological invariants are elaborated for identifying the
deformation classes of real Enriques surfaces. Some of theses are new
and can be applied to other classes of surfaces or higher-dimensional
varieties. Intended for researchers and graduate students in real
algebraic geometry it may also interest others who want to become
familiar with the field and its techniques. The study relies on topology
of involutions, arithmetics of integral quadratic forms, algebraic
geometry of surfaces, and the hyperkähler structure of K3-surfaces. A
comprehensive summary of the necessary results and techniques from each
of these fields is included. Some results are developed further, e.g., a
detailed study of lattices with a pair of commuting involutions and a
certain class of rational complex surfaces.