The aim of this research is to develop a measurement instrument that is valid and reliable, called the Middle School-Life Science Concept Inventory (MS-LSCI), for the purpose of measuring the life science conceptual understanding of middle school-level students. Although there are several existing concept inventories related to biology concepts (i.e. Secondary-Biology Concept Inventory (S-BCI) and Biology Concept Inventory (BCI)), there is no fully developed concept inventory available that collectively measures the major life science concepts covered in middle school classrooms (Klymkowsky, Underwood, & Garvin-Doxas, 2010; Stammen, Lan, Schuchaerdt, Malone, Ding, Sabree, & Boone, 2016). Study one focuses on how data from a multi-panel expert review and student interviews were used in the Middle School-Life Science's (MS-LSCI) item content qualitative validation and iterative refinement process. Of the 50 questions reviewed by the expert panels, 12 items were identified as having content validity concerns. Generally, these content validity concerns fell within two categories: (i) imprecise phrasing and (ii) age inappropriateness. During the student interviews, a total of 26 items were identified as displaying content validity issues. These 26 items fell into one of three categories: (i) imprecise phrasing, (ii) contextual ambiguity, and (iii) formatting/diagrammatic complexity. Using the data from the multi-panel expert review and student interviews, the items with content validity concerns were refined and modified before the items were field tested. Study two describes the MS-LSCI's quantitative validation and item selection process. Specifically, this study focuses on the psychometric functioning of the 60 field-tested MS-LSCI items using Rasch analysis. The results of this development, refinement, and evaluation process suggest that the 25-item MS-LSCI is a valid instrument in that the items appear to be unidimensional, item and person measures display little misfit, and the reliability values suggest replicability within the targeted sample. Based on these results, the MS-LSCI has the potential to help fill the gap in the assessment tools available to measure middle school student life science concepts in conjunction with alternative conceptions. Study three describes the difference among 6th, 7th, and 8th graders' performance on the MS-LSCI's 60 field tested items. The results suggest that MS-LSCI performance was significantly lower for 6th graders relative to 8th graders and that six items can be linked to this difference in performance. These six items fell within the MS-LSCI's core concepts of evolution and diversity, population interaction, and growth and reproduction. More importantly, these six items shed light on the contextual differences between 6th and 8th grade performance. The contextualized item differences discovered in this study may be attributed to middle school life science curriculum sequencing and the perseverance of specific alternative conceptions. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]