Magnetic resonance imaging (MRI) and computed tomography (CT) are the criterion imaging methods used to estimate skeletal muscle mass (SMM). Ultrasonography (US) is a portable imaging technique that can be used to quantify skeletal muscle thickness (MT) and subsequently, estimate SMM. Estimates of SMM obtained using US are strongly correlated with estimates obtained using MRI (r=0.53-0.96). US has been used to predict cross sectional area in comparison to CT (r=0.911).Compared to MRI and CT, US operates at a lower cost and does not require a radiation dose. Although widely used, there are several methodological inconsistencies in the measurement of MT using US and therefore, accuracy and precision of the measure remains unknown. Methodological issues include the standardisation of probe size, the anatomical landmarks used to determine the measurement site, the process of image capture, the extent to which assessors are blind and the time frame between repeated measures. (1-2) Following a review of literature a standardised operating procedure (SOP) was developed for US to; Standardise the process of image capture Standardise the measurement of MT The aim of this pilot study was to investigate the inter and intra-rater reliability and standard error of measurement (SEM) associated with a newly established SOP for the measurement of vastus lateralis MT. Method: A pilot study was conducted on 10 healthy adults (5 males; 5 females) from the musculoskeletal health research group at Leeds Beckett University. Two investigators independently measured vastus lateralis MT (right leg) using a SOP. Investigators, independently, located and marked the measurement site. Three images were captured and measured onscreen. Investigators were blinded to each individual measurement. Data was analysed in SPSS, Intra-class Correlation Coefficient (ICC) was calculated for both inter and intra rater reliability. Results: Inter-rater and intra-rater reliability was high for the measurement of vastus lateralis MT. Inter-rater reliability ranged from 0.959-0.987 (standard deviation(s)= 0.901-0.974, 95% CI, P <0.05) for the three trials. Intra-rater reliability ranged from 0.960-0.983 (s=0.944-0.978, 95% CI, P<0.05) for investigator one and 0.976-0.989 (s=0.898-0.962, 95% CI, P<0.05) for investigator two. The SEM between investigators was 0.16cm. The SEM for investigator one was 0.16cm and for investigator two was 0.12cm. Conclusion: Two raters achieved excellent levels of inter and intra-rater reliability when quantifying vastus lateralis MT using a newly established SOP. In this small sample, the results demonstrate that excellent levels of agreement can be reached when raters independently locate and mark the MT measurement site, capture and measure three separate images and are blind to the measurements recorded.