The crystalline state is the most stable form of all solids and is very common in nature. This book discusses the structure of crystals and the ways in which they are described. The basic ideas of symmetry are introduced first. The lattice types which are possible are then examined and this leads to the fourteen Bravais lattices. There follows a careful treatment of the convolution operation and its applications to crystals and the calculation of Fourier transforms. These ideas allow crystal structure to be determined from the results of diffraction experiments. The effects of the thermal vibration of atoms and finite crystal size on these experiments is also explained. Throughout the text the emphasis is on understanding the results rather than on rigorous proofs.