Background: Residual limb volume is often measured as part of routine care for people with amputations. These measurements assist in the timing of prosthetic fitting or replacement. In order to make well informed decisions, clinicians need access to measurement tools that are valid and reliable. Objectives: To assess the reliability and criterion validity of the iSense optical scanner in measuring volume of transtibial residual limb models. Study Design: Three assessors performed two measurements each on 13 residual limb models with an iSense optical scanner (3D systems, USA). Intra-rater and inter-rater reliability were calculated using intraclass correlation coefficients. Bland Altman plots were inspected for agreement. Criterion validity was assessed using a steel rod of known dimensions. Ten repeated measurements were performed by one assessor. A t-test was used to determine differences between measured and true rod volume. Results: Intra-rater reliability was excellent (range of intraclass correlation coefficients: 0.991���0.997, all with narrow 95% confidence intervals). While the intraclass correlation coefficients suggest excellent inter-rater reliability between all three assessors (range of intraclass correlation coefficients: 0.952���0.986), the 95% confidence intervals were wide between assessor 3 and the other two assessors. Poor agreement with assessor 3 was also seen in the Bland-Altman plots. Criterion validity was very poor with a significant difference between the mean iSense measurement and the true rod volume (difference: 221.18 mL; p < 0.001). Conclusions: Although intra-rater reliability was excellent for the iSense scanner, we did not find similar results for inter-rater reliability and validity. These results suggest that further testing of the iSense scanner is required prior to use in clinical practice. Clinical relevance: The iSense offers a low cost scanning option for residual limb volume measurement. Intra-rater reliability was excellent, but inter-rater reliability and validity were such that clinical adoption is not indicated at present.