This is the first lecture in a graduate level course entitled Diffraction Methods (Chem 7340) at Ohio State University. In this lecture we consider the differences between crystalline and amorphous solids, recognizing that translational symmetry is the characteristic feature that defines crystalline solids. We then examine translational symmetry in two dimensional space, and introduce the five 2D Bravais lattices. The lecture finishes by showing how crystals can be built up by combining a group of atoms (the motif) with each lattice point. The lecture and those that follow are created and delivered by Dr. Patrick Woodward.