  In this paper the Wiener--Hopf factorisation problem is presented in a
unified framework with the Riemann--Hilbert factorisation. This allows to
establish the exact relationship between the two types of factorisation. In
particular, in the Wiener--Hopf problem one assumes more regularity than for
the Riemann--Hilbert problem. It is shown that Wiener--Hopf factorisation can
be obtained using Riemann--Hilbert factorisation on certain lines.
