The primary purposes of this article are to review the literature on seating assessment and to describe the development of a clinical evaluation scale, the Seated Postural Control Measure (SPCM), for use with children requiring adaptive seating systems. The SPCM is an observational scale of 22 seated postural alignment items and 12 functional movement items, each scored on a four-point, criterion-referenced scale. A secondary purpose of this article is to report the reliability of the seven-point Level of Sitting Scale (LSS). Interrater and test-retest reliability of the SPCM items and the one-item LSS were evaluated on a sample of 40 children with developmental disabilities who sat with and without their seating systems. Kappa values of .75 or higher were considered excellent, .40 to .74 as fair to good, and less than .40 as poor. The interrater reliability tests for the two seated conditions and the two test sessions conducted 3 weeks apart yielded overall item Kappa coefficient means of .45 for the alignment section and .85 for the function section. Test-retest results for the SPCM items were less satisfactory, with item Kappa coefficient means for the two seating conditions and raters of .35 and .29 for alignment and function, respectively. Reliability results did not appear to be consistently better among seating conditions, raters, or test sessions. Kappa coefficients for the LSS were fair to good for both interrater and test-retest reliability. Plans for future development of the SPCM and LSS are discussed.