Random walk in random environment (RWRE) is a fundamental model of statistical mechanics, describing the movement of a particle in a highly disordered and inhomogeneous medium as a random walk with random jump probabilities. It has been introduced in a series of papers as a model of DNA chain replication and crystal growth (see Chernov [Ch67] and Temkin [Te69, Te72]), and also as a model of turbulent behavior in fluids through a Lorentz gas description (Sinaȋ 1982 [Si82a]). It is a simple but powerful model for a variety of complex large-scale disordered phenomena arising from fields such as physics, biology and engineering. While the one-dimensional model is well-understood, in the multidimensional setting, fundamental questions about the RWRE model have resisted repeated and persistent attempts to answer them. Two major complications in this context stem from the loss of the Markov property under the averaged measure as well as the fact that in dimensions larger than one, the RWRE is not reversible anymore. In these notes we present a general overview of the model, with an emphasis on the multidimensional setting and a more detailed description of recent progress around ballisticity questions.We will usually call the environment (uniformly) elliptic in that case also.Remark 2.4. This labeling is motivated by operator theory where one has analogous definitions of elliptic and uniformly elliptic differential operators.The following auxiliary process will play a significant role in what follows.Definition 2.5. (Environment viewed from the particle). Let (X n ) be a RWRE. We define the environment viewed from the particle (or also the environmental process) as the discrete time processω n := t Xn ω, for n ≥ 0, with state space Ω.Apart from taking values in a compact state space, another advantage of the environment viewed from the particle is that even under the averaged measure it is Markovian, as is shown in the next result following Sznitman [BS02]; however, the cost is that we now deal with an infinite dimensional state space.Proposition 2.6. Consider a RWRE in an environment with law P. Then, under P 0 , the process (ω n ) is Markovian with state space Ω, initial law P, and transition kernel Rf (ω) := e∈U ω(0, e)f (t e ω), (2.8) defined for f bounded measurable on Ω and initial law P. Proof. Let us first note that for every x ∈ Z d , and every bounded measurable function f on Ω, E x,ω [f (ω 1 )] = E x,ω [f (t X 1 ω)] = e∈U ω(x, e)f (t x+e ω) = e∈U t x ω(0, e)f (t e (t x ω)) = Rf (t x ω).(2.9)