Due to the problem-centric nature of its mandate, empirical research has been relatively central in the United Nations Commission on International Trade Law (UNCITRAL) investment arbitration reform process. In this article, the authors seek to provide a state-of-the-art summary and assessment of empirical studies on the six identified concerns of states: legal cost, duration of proceedings, consistency, correctness, diversity and independence. The article asks: (1) What do we know? and (2) Does it matter? The survey of evidence reveals an emerging base of quantitative, qualitative and computational evidence for justifying some but not all concerns and understanding their causes. However, there are challenges in accessing all relevant data, modelling outcomes and evaluating whether there was normatively a problem. The article concludes by indicating that some concerns are clearly justified, others not, and others fall within an unknown category.