The quality-by-design (QbD) approach is widely utilized for developing and validating manufacturing processes for drug substances as well as drug products. This paper discusses the application of the risk-based QbD approach used at F. Hoffmann-La Roche Ltd. for development, optimization, and characterization of drug substance manufacturing processes for small molecules. It presents the evolution of the QbD concept into statistical thinking and development of a quantitative tool, namely, the impact ratio concept, for its successful implementation. The utilization of this approach is illustrated with a case study from the taselisib drug substance manufacturing process. KEYWORDS: quality by design (QbD), quality risk assessment (QRA), process characterization, design of experiment (DoE), critical process parameter (CPP), critical material attribute (CMA), critical quality attribute (CQA), statistics, impact ratio (IR), process performance qualification (PPQ) Article pubs.acs.org/OPRD