This document discusses the design of fragment screening libraries for fragment-based drug discovery. It describes how fragment hits have high ligand efficiency due to their low molecular weight. The document outlines criteria for selecting fragments, including molecular complexity, solubility, and availability of chemical neighbors. It presents details on the design of the GFSL05 20,000 compound screening library from AstraZeneca, including controlling molecular properties like size, lipophilicity, and structural diversity. Literature on fragment-based screening and library design is also cited.