Scientific literacy reflects "a broad and functional understanding of science for general education purposes" (DeBoer, p. 594). Herein, we present the ongoing development of the Scientific Literacy Assessment (SLA), a work-in-progress measure to assess middle school students' (ages 11-14) scientific literacy. The SLA includes a selected response measure of students' demonstrated scientific literacy (SLA-D) and a motivation and beliefs scale based on existing measures of self-efficacy, subjective task value, and personal epistemology for science (SLA-MB). Our theoretical conceptualization of scientific literacy guided the development of our measure. We provide details from three studies: Pilot Study 1 (n = 124) and Pilot Study 2 (n = 220) describe the development of the SLA-D by conducting iterative item analyses of the student responses, think-aloud interviews with six students, and external expert feedback on the items in the SLA-D. Study 3 describes the testing of our prototype measure (n = 264). We present a validity argument including reliability evidence that supports the use of the current version of the SLA to provide evaluation of middle school students' scientific literacy. Our resulting SLA includes the SLA-D in two versions, each with 26 items and the SLA-MB with 25 items across three scales: value of science, scientific literacy self-efficacy, and personal epistemology.