This is a 3 hours lecture I was asked to give at the 13th CNRS Thematic School on Computational Fluid Dynamics. This year was dedicated to Multi-Scale Methods and Applications. This lecture was thought as an introduction to micro/nano-fluidics from the physical and numerical viewpoints. We talked about "microscopic" and "continuum" models and links between them. Various numerical methods (DSMC, MD, DPD) are introduced successively by considering gases then liquids. In the last part, we made an illustration of application by focusing on the problem of wall law for fluids which has a rich history and is a good prototype of a multiscale problem for which convergence of experimental, physical and mathematical modelling approaches allows to give insight on slippage phenomena. This lecture emphasized on microfluidics and aimed at forming a partition as well as making links with other lectures of the conference from Dominique d'Humières on Lattice Boltzmann Methods and Petros Koumoutsakos on Coupling between Discrete and Continuum Numerical Methods.