Syst mes multi- chelles is an introduction to the modelling and the simulation of multiscale problems, which are now ubiquitous in many fields of scientific computing: materials science, fluid mechanics, molecular simulation, chemical engineering, ... The standard techniques, such as homogeneization techniques, are presented, along with the most recent and advanced ones, such as the micromacro modelling of non-newtonian fluids. Mathematical objects and theories as diverse as partial differential equations, stochastic differential equations, calculus of variations, are introduced and manipulated