This article describes the development of the Early Elementary Writing Rubric (EEWR), an analytic assessment designed to measure kindergarten and first-grade writing and inform educators' instruction. Crocker and Algina's (1986) approach to instrument development and validation was used as a guide to create and refine the writing measure. Study 1 describes the development of the 10-item measure (response scale ranges from 0 = "Beginning of Kindergarten" to 5 = "End of First Grade"). Educators participated in focus groups, expert panel review, cognitive interviews, and pretesting as part of the instrument development process. Study 2 evaluates measurement quality in terms of score reliability and validity. Data from writing samples produced by 634 students in kindergarten and first-grade classrooms were collected during pilot testing. An exploratory factor analysis was conducted to evaluate the psychometric properties of the EEWR. A one-factor model fit the data for all writing genres and all scoring elements were retained with loadings ranging from 0.49 to 0.92. Internal consistency reliability was high and ranged from 0.89 to 0.91. Interrater reliability between the researcher and participants varied from poor to good and means ranged from 52% to 72%. First-grade students received higher scores than kindergartners on all 10 scoring elements. The EEWR holds promise as an acceptable, useful, and psychometrically sound measure of early writing. Further iterative development is needed to fully investigate its ability to accurately identify the present level of student performance and to determine sensitivity to developmental and instruction gains.