The Toolkit on using Data Science for Competition Assessment provides a practical guide for competition authorities and regulators on the quantitative tools and techniques available to gather, clean and analyze data in competition cases, studies, and impact assessments. The toolkit discusses the following: (i) techniques that can be used to gather data through merger notification templates, web scrapping techniques, accessing government information, market studies, and surveys; (ii) methods to ensure data quality before these are analyzed by regulators; (iii) empirical techniques that can be used to analyze typical competition issues such as defining markets, assessing market power, predicting outcomes of horizontal mergers, identifying collusion, and determining appropriate penalties for anticompetitive conduct; and (iv) strengths and limitations on the use of data science tools.