This chapter provides a self-contained and thorough introduction to the continuum Gaussian free field (GFF) with zero (or Dirichlet) boundary conditions. We start by describing its discrete counterpart, before presenting two constructions of the continuum object: one as a stochastic process, and the other as a random generalised function. We explain the equivalence of these two perspectives, and in the remainder of the chapter, draw on both viewpoints to prove various important properties. In particular, we prove that the GFF satisfies a certain domain Markov property and exhibits precise scaling behaviour. In two dimensions, this is a special case of its (more general) conformal invariance. We go on to study the so-called thick points of the GFF in two dimensions, which are fractal sets of points where the field is atypically “high” and are particularly useful for understanding the Gaussian multiplicative chaos measures associated with the GFF in later chapters. We close the initial chapter with a rigorous scaling limit result, justifying that the continuum GFF is indeed the scaling limit of its discrete counterpart.