This study is concerned with how teachers use data about students' ideas in science, elicited from embedded formative assessment probes, to make decisions about next steps in instruction. Since Black and Wiliam's (1998) seminal work on formative assessment, researchers and practitioners alike concerned themselves with formative assessment and its potential benefits to instruction and student achievement. However, the literature shows that a significant hindrance for educators is their ability to plan next steps in instruction in response to data elicited from formative assessment. This study drew upon the concept of pedagogical design capacity and the Design Capacity for Enactment framework (Brown, 2002) as a means to better understand what resources teachers both identify and take advantage of when making decisions about formative assessment outcomes. The study took a qualitative approach and employed a case study methodology. I engaged two fourth grade teachers at an independent laboratory school in Pittsburgh. These teachers implemented two different formative assessment probes twice. The study collected data from five sources. Observations with video and audio recordings captured teachers working with information elicited from probes in planning sessions as well as planning documents dedicated to science. Teachers annotated unit and lesson plans. Lastly, teachers were interviewed individually multiple times throughout the study and at the end. Analysis of the data collected utilized analytic memo writing and first and second cycle coding. The study found that teachers drew heavily on personal resources (pedagogical content knowledge, subject matter knowledge, and beliefs and goals) when making instructional decisions about next steps in instruction. Teacher response to data from the formative assessment probes became increasingly responsive to student misconceptions as the unit progressed. Early in the animal unit not many changes were made; later, every lesson was adapted or improvised in response to the data. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]