Envisioned by Richard Feynman in the early 1980s, quantum simulation has received dramatic impetus thanks to the development of a variety of plateforms able to emulate a wide class of quantum Hamiltonians. During the past decade, most of the quantum simulators have implemented rather well-known models, hence permitting a direct comparison with theoretical calculations and a precise benchmarking of their reliability. The field has now reached a maturity such that one can address difficult problems, which cannot be solved efficiently using classical algorithms. These advances provide unprecedented opportunities to explore previously unreachable fields, test theoretical predictions, and inspire novel approaches.This contribution is an elementary introduction to quantum simulation. We discuss the challenges, define both digital and analog quantum simulators, and list the demanding conditions they require. We also provide a brief account of the contributions gathered in the dossier on Quantum Simulation of the Comptes-Rendus de Physique of the French Academy of Sciences [1,2,3,4,5,6]. The latter completes excellent reviews that appeared previously, see for instance Refs. [7,8,9,10,11,12,13,14].Universal models and the role of simulations in many-body physics Understanding the behavior of macroscopic quantum systems is a major challenge of modern physics. The basic laws of low-energy physics are by now quite well known at the microscopic, say atomic, scale. Conversely, many fundamental questions remain open, and even debated, about the collective dynamics at the macroscopic scale. By "macroscopic scale", here we mean systems made up of a huge number of constituents, or degrees of freedom, say 10 6 , 10 23 , or even more, as relevant in condensed matter physics or in astrophysics, for instance. Such huge systems cannot be treated exactly, be it a the classical level and, even worse, at the quantum level. Yet, it is the main outcome of the thermodynamic approach that the collective behavior of a macroscopic system can drastically differ from that of its elementary constituents. For instance, the elementary interactions between the H 2 O molecules are fundamentally unchanged when a water bucket turns from the liquid phase to the solid phase at zero degree Celsius. Similarly, the interactions between the microscopic magnetic moments do not show any brutal change when a magnetic material gets magnetized underneath the Curie temperature. Hence, the dramatic effects observed in macroscopic systems are governed by largescale instabilities, without obvious counterparts at the microscopic level. This observation takes a universal character, summarized in the celebrated motto "More is Different" [15]. Such so-called emerging phenomena also appear in quantum systems, where new effects that are impossible in the classical world show up below some critical temperature or at zero temperature when some interaction parameter passes through a critical value. Celebrated examples include the superfluid Λ transition in helium or super-cond...