The timed 'Up & Go' test (TUG) is a test of basic or functional mobility in adults which has rarely been used in children. Functional mobility was defined for this study as an individual's ability to manoeuvre his or her body capably and independently to accomplish everyday tasks. Reliability and validity of TUG scores were examined in 176 children without physical disabilities (94 males, 82 females; mean age 5y 9mo [SD 1y 8mo]; range 3 to 9y) and in 41 young people with physical disabilities due to cerebral palsy or spina bifida (20 males, 21 females; mean age 8y 11mo [SD 4y 3mo], range 3 to 19y). Mean TUG score for children without physical disability was 5.9s (SD 1.3). Reliability of the TUG test was high, with intraclass correlation coefficients (ICC) of 0.89 within session, and 0.83 for test-retest reliability. Mean score of the group aged 3 to 5 years was significantly higher (6.7s SD 1.2) than that of the older group (5.1s, SD 0.8; p=0.001). Scores in the younger group reduced significantly over a 5-month follow-up period (p=0.001), indicating that the TUG was responsive to change. Within-session reliability of the TUG in young people with disabilities was very high (ICC=0.99). There were significant differences in TUG scores between children classified at levels I, II, and III of the Gross Motor Function Classification System (p=0.001). TUG scores showed a moderate negative correlation with scores on the Standing and Walking dimensions of the Gross Motor Function Measure (n=22, rho=-0.52, p=0.012). There was no significant difference in TUG scores between typically developing male and female children. The TUG can be used reliably in children as young as 3 years using the protocol described in this paper. It is a meaningful, quick, and practical objective measure of functional mobility. With further investigation, the TUG is potentially useful as a screening test, an outcome measure in intervention studies for young people with disabilities, a measure of disability, and as a measure of change in functional mobility over time.