These notes are based on a talk given at the 2018 Arizona School of Analysis and Mathematical Physics. We give a comprehensive introduction to the KPZ universality class, a conjectured class of stochastic process with local interactions related to random growth processes in 1 + 1 dimensions. We describe some of the characteristic properties of the KPZ universality class such as scaling exponents and limiting statistics. In particular, we aim to extract the characteristic properties of the KPZ universality class by understanding the KPZ stochastic partial differential equation by a special discrete approximation given by the asymmetric simple exclusion process (ASEP). The connection with the ASEP is very important as the process enjoys a rich integrability structure that leads to many exact formulas.Contents arXiv:1904.03319v1 [math-ph] 5 Apr 2019 with W (t, x) a Gaussian space-time white noise random variable (i.e. a δ-correlated stationary, Gaussian process with mean zero and covariance E [W (t , x )W (t, x)] = δ(t − t)δ(x − x)). The first term ν∂ 2x h(t, x) is a diffusion term which tends to smooth out the height function as time evolves, and the term λ 2 (∂ x h(t, x)) 2 is a lateral growth term, which makes the surface grow normal to its interface. The SPDE defined by eqn. (1.1) is now called the KPZ equation.It turns out that solving the KPZ equation is highly non-trivial, starting with making the KPZ equation (1.1) well-defined. For instance, through a local scaling argument, one might expect that the height function behaves similar to a Brownian motion due to the space-time white noise, and this means that, a priori, the term ∂ x h(t, x) is a distribution without a well-defined square (∂ x h(t, x)) 2 making the KPZ equation ill-defined. Under certain initial conditions, this issue has been resolved in the work of [BG97, ACQ11] by introducing the Cole-Hopf transformation (see eqn. 2.2) and an appropriate discrete approximation of the KPZ equation, and also, in the work of [Hai11] through the theory of rough paths. In [BG97], Bertini and Giacomoni gave a solution to the KPZ equation for initial conditions h(0, x) that (in average) grow at most linearly, such as flat initial conditions or two-sided Brownian motion initial conditions. In particular, Bertini and Giacomoni approach the KPZ equation by first smoothing out the noise, solving the smoothed KPZ equation via a discrete approximation using a discrete Cole-Hopf transformation, and finally taking the appropriate limits to a solution of the KPZ equation. Then, in [ACQ11], Amir, Corwin and Quaste were able to extend the results of [BG97] by treating the KPZ equation with the infinite narrow wedge initial condition (i.e. h(0, x) = log(δ x=0 )), which is a natural initial condition under the random polymer interpretation of the KPZ equation. In [ACQ11], the authors still use the idea of considering the Cole-Hopf transformation and were able to treat the wedge initial conditions via the exact formulas, introduced by Tracy and Widom in [TW08, TW09], for the asy...