Concerto: An Open Source Online Adaptive Testing Platform. The University of Cambridge Psychometrics Centre’s mission is to encourage the use of high quality assessments in business, health and education. We believe that 90% of online tests could one day be created without needing to write a single line of code, why is why we’ve built Concerto: a powerful and user-friendly platform that empowers experts and beginners alike to make better tests. Concerto harmonises the statistical power of the R programming language, the security of MySQL databases and the flexibility of HTML to deliver an outstanding performance. These instruments work in unison, giving users unparalleled freedom and control over the design of their assessments. In-built algorithms for score calculation and report generation ensure a rewarding experience for participants, whatever the context. Most importantly, Concerto is open source and completely free to use. Anyone can create a functioning test in minutes by simply connecting elements in a flowchart. There are no set-up costs, no hidden licence fees and no limitations