SPIP: a computer program implementing the interaction picture method for simulation of light-wave propagation in optical fibre. The computer program spip is aimed at solving the Generalized Non-Linear Schr”odinger equation (GNLSE), involved in optics e.g. in the modelling of light-wave propagation in an optical fibre, by the Interaction Picture method, a new efficient alternative method to the Symmetric Split-Step method. In the spip program a dedicated costless adaptive step-size control based on the use of a 4th order embedded Runge-Kutta method is implemented in order to speed up the resolution.