  In this work we study the existence of invariant almost complex structures on
real flag manifolds associated to split real forms of complex simple Lie
algebras. We show that, contrary to the complex case where the invariant almost
complex structures are well known, some real flag manifolds do not admit such
structures. We check which invariant almost complex structures are integrable
and prove that only some flag manifolds of the Lie algebra $ C_{l}$ admit
complex structures.
