The Florida Center for Research in Science, Technology, Engineering and Mathematics (FCR-STEM) was awarded a grant by the Florida Department of Education to develop a Mathematics Formative Assessment System (MFAS) aligned with the Common Core State Standards (CCSS). Intended for both teachers and students, formative assessment is a process that provides feedback to adjust ongoing teaching and learning to improve students' achievement of instructional goals. The evidence collected enables teachers to differentiate instruction based on students' mathematical thinking and reasoning rather than solely on incorrect answers. The MFAS-CCSS was designed to provide teachers with tasks and rubrics to employ the following five key strategies: (1) assess the student's level of understanding during instruction; (2) identify the student's specific misconceptions and errors; (3) examine samples of student work for further evidence of student understanding; (4) pose additional questions to elicit student thinking; and (5) obtain guidance on next steps for instruction. The current study--the MFAS-CCSS 2012-13 Study--is the third randomized field trial conducted on MFAS. The purpose was to evaluate the effect the MFAS-CCSS has on teachers' knowledge of elementary mathematics and students' mathematics learning in Grades K-1. The MFAS-CCSS 2012-13 Study made use of intervention protocols and measures developed, field tested, and validated in the prior two pilot studies. Two tables are appended.