Stage theories of spelling development have been widely accepted since their conceptualisation in the 1970s; but, converging evidence offers support for non-linear models of spelling development. Specifically, Triple Word Form Theory (TWFT) suggests that students are capable of drawing on and coordinating phonological, orthographic, and morphological skills from quite early in their spelling development. Assumptions about the nature of spelling development have important implications for the way spelling is taught; however, many existing assessment systems currently being applied by educators and systems have failed to consider non-linear perspectives of spelling development. This paper presents the Components of Spelling Test (CoST) as a new approach to spelling assessment. The test is a valid and reliable assessment tool informed by TWFT and developed through a method of analysis suggested by stage theorists. Data used to develop and test the CoST were drawn from students in Year 3 and Year 5 across four schools in the Australian Capital Territory (ACT) (n=198). The results of the testing clearly demonstrated that the CoST can provide teachers and educational researchers with a means to validly and reliably identify individual differences in specific phonological, orthographic and morphological skills associated with spelling.