The focus of this chapter is on the Teacher Learning Instrument (TLI), a metacognitive tool to deepen understanding of literacy teaching and heighten awareness of the teacher's role in student learning. Four questions are addressed in this chapter: (1) What is the TLI? (2) How does the TLI engage teachers in metacognitive inquiry? (3) What are considerations in using the TLI? (4) What are directions for further research on inquiry tools for professional development? The first section describes the theoretical roots of the TLI and highlights findings from research on the use of the TLI in various educational settings. The second section illustrates through examples of transcript data the metacognitive processes and substantive dialogue that participants engage in when using the instrument. The third section closes the chapter with considerations for using the TLI and suggestions for further research.