SHELXL

Abstract This chapter introduces the crystal structure refinement program SHELXL, a program written and maintained by George M. Sheldrick. SHELXL is by far the most popular refinement program for small molecules and, together with CNS by Axel Brünger and Refmac by Garib Murshudov, it is one of the three most commonly used programs for the refinement of protein structures. The chapter describes the basic program organization of SHELXL as well as the input and output file structure. It also explains the concept of the connectivity table.