Students do their best work when they have a sense of agency around science and science learning. Scientific practices like argumentation and critique are areas in which students' agency is especially visible, as they require many points of decision-making and evaluation. While argumentation and critique are fundamental practices in both classroom and professional science, these practices can remain contrived if students do not have an agentic relationship to them. This study investigated two aspects of support for students' agency, especially as it relates to critique. First, a case study in a 5th grade classroom uncovered the criteria (e.g., standards) which the students and their teacher used to make decisions, as well as how those criteria were negotiated over the course of a unit. Findings suggested that students used diverse criteria which fluctuated depending on the task and object of critique, and that both parties' negotiation for influence was mutually supportive for learning. Second, an invariance study aimed to advance the measurement quality around teachers' support for student agency, especially in measuring their growth over time. Using invariance anlaysis, an appropriate measurement model was generated so that teachers' scores could be compared across time by using a similar measuring stick at each occasion. Findings highlighted areas where response shifts occurred and the measuring sticks were not comparable, ultimately advancing knowledge of how teachers interpret items related to agency and improving efforts to measure teachers' support for student agency. Overall, students' sophisticated use of criteria plays an important role in the extent to which they have a sense of agency around science learning. Teachers' own sense of agency around science may impact how they support students, and efforts to measure teachers' support for student agency might benefit from attention to these issues. [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.]