Normal selective voluntary motor control (SVMC) can be defined as the ability to perform isolated joint movement without using mass flexor/extensor patterns or undesired movement at other joints, such as mirroring. SVMC is an important determinant of function, yet a valid, reliable assessment tool is lacking. The Selective Control Assessment of the Lower Extremity (SCALE) is a clinical tool developed to quantify SVMC in patients with cerebral palsy (CP). This paper describes the development, utility, validation, and interrater reliability of SCALE. Content validity was based on review by 14 experienced clinicians. Mean agreement was 91.9% (range 71.4-100%) for statements about content, administration, and grading. SCALE scores were compared with Gross Motor Function Classification System Expanded and Revised (GMFCS-ER) levels for 51 participants with spastic diplegic, hemiplegic, and quadriplegic CP (GMFCS levels I - IV, 21 males, 30 females; mean age 11y 11mo [SD 4y 9mo]; range 5-23y). Construct validity was supported by significant inverse correlation (Spearman's r=-0.83, p<0.001) between SCALE scores and GMFCS levels. Six clinicians rated 20 participants with spastic CP (seven males, 13 females, mean age 12y 3mo [SD 5y 5mo], range 7-23y) using SCALE. A high level of interrater reliability was demonstrated by intraclass correlation coefficients ranging from 0.88 to 0.91 (p<0.001).