  I present an extension of the thin layer approximation to non-axisymmetric
bow shocks. By choosing a suitable set of curvilinear coordinates that matches
the geometry of a generally distorted bow shock surface, I derive the fluid
equations for the flow. Analytical expansions are given for the matter flow
near the bow shock stagnation point. Numerical solutions are also obtained, by
using a revised Runge-Kutta method. This numerical method results to be
computationally stable and fast. When two winds interact, axisymmetric bow
shocks are expected to form only if both winds are spherically symmetric. But
stellar winds often present strong anisotropies: for all these cases a
non-axisymmetric model of bow shock is required.
