Fragment-based drug discovery (FBDD) is complementary to high-throughput screening. The approach has two key stages: identifying the starting fragment hit to be developed and generating the lead compound from the starting fragment hit. Here, we provide an overview of FBDD and introduce two indices originally developed at Astellas Pharma. The first is related to the size ratio of fragment hits to drug leads; this is useful for fragment-library design and the fragment-to-lead process. The second is related to maximum ligand efficiency; this is useful for fragment hit prioritization and the fragment-to lead process. Both indices are based on the ‘Golden Ratio’.