Different approximations for the description of the internal stress fi
eld due to dislocations walls are given and discussed. The influence o
f disorder and the edge effects due to incomplete walls are particular
ly discussed. Finally the relationship between the internal stress and
the flow stress are discussed in these particular cases. It is shown
that identifying the flow stress with the internal stress averaged in
the volume of the crystal may be misleading. Consequences on the propa
gation of glide bands are also examined.