This chapter summarises the rationale for the development and validation work that took place over 2.5 years before the launch of the computer-based (CB) format of the Cambridge English Young Learners English tests (YLE). Several rounds of trials were carried out in a cyclical way, in a number of different locations across various countries, to ensure data was collected from a representative sample of candidates in terms of geographical location, age, L1, language ability, familiarity with the YLE tests, and experience of using different computer devices – PC, laptop and tablet. Validity evidence is presented from an empirical study, using a convergent mixed methods design to explore candidate performance in and reaction to the CB YLE tests. Regression analyses were conducted to investigate which individual test taker characteristics contribute to candidate performance in CB YLE tests. The results indicate that CB delivery presents a genuine choice for candidates in line with the Cambridge English ‘bias for best’ principle. Positive feedback from trial candidates, parents and examiners suggests that CB YLE tests offer a contemporary, fun, and accessible alternative to paper-based (PB) YLE tests to assess children’s English language ability.