This chapter reviews the current theoretical and experimental picture of the vortex-lattice structure in conventional and high-Tc superconductors. The volume of published work on this topic is large. The purpose of this chapter is not to provide an exhaustive compilation of these studies, but rather to highlight the most significant developments through a discussion of selected references. The initial part of this chapter discusses the general structure of the vortex lattice and the corresponding field distribution, and how this structure is affected by pinning, thermal fluctuations and anisotropy. Next, the problems associated with modelling the field distribution with a simple Gaussian function are discussed. The second half of the chapter concerns itself with a more accurate way of modelling the field distribution associated with the vortex lattice. In particular, the vortex-lattice structure in both an s-wave and a dx2-y2-wave superconductor are discussed in some detail. In addition, vortex-lattice imaging experiments in both conventional and high-Tc superconductors are reviewed.