The MILESTONE project developed the Transition Readiness and Appropriateness Measure (TRAM) and the Transition-Related Outcome Measure (TROM) on the HealthTracker™ platform, each prepared in versions for young people, parents/carers and clinicians. Together these instruments aim to support and then evaluate clinician decision-making with respect to transition. The suite of measures were developed on and hosted on the HealthTracker™ Platform. FDA approved protocols were evoked in scale development and validation. A comprehensive list of items potentially significant in transition decision-making was generated from a thorough literature review and discussion with experts. Focus groups were conducted with young people, parents/carers and clinicians centring on the themes of “who should transition” and “identifying successful transition”. In open discussion, further items considered important in transition decision-making were elicited, and the importance of listed items was rated. Analysis of the data allowed items to be removed, kept or amalgamated. Domains considered universally important in transition decision-making emerged; these included diagnosis, impairment, risk, life changes, barriers to a successful transition and transition success markers. A beta version of the scale was tested for comprehension and usability by transition experts, young people and parents/carers. Following pilot testing, qualitative interviews were conducted with some participants to identify further issues. Scales were translated from English into French, Italian, German, Croatian and Dutch and translations uploaded to the HealthTracker™ online platform. Validation of the scales required completion of the TRAM and TROM alongside a series of proxy “gold-standard” measures to assess psychometric validity, test-retest validity and sensitivity to change.