Teacher beliefs are implicit and explicit suppositions held by educators which have relevance for their professional and instructional practices, interactions with students, and learning processes. They may include beliefs about students, self, learning, knowledge, and knowing. Beliefs about knowledge and knowing—teacher epistemologies—are a specific and important type of teacher belief. Teacher beliefs and epistemologies merit investigation given their influence on teaching practices and student learning, yet the form, nature, development, and propensity to change with respect to these constructs are open to question. Further, their expression in teachers’ practice is complex, not least because both the construct and content of “belief” and “epistemology” are equivocal, elaborate, and closely connected to other constructs. Much of the teacher beliefs and epistemology research has emerged from the field of educational psychology. This chapter describes five central issues related to teachers’ beliefs and epistemologies during recent decades. Similar core issues were identified in the Oxford Research Encyclopedias in Education article “Reviews of Teachers’ Beliefs.” Within each of five sections, which represent the core issues in the field, we first explore teacher beliefs in general followed by teacher epistemologies specifically. In the first section, we provide general overviews of teacher beliefs and epistemologies to introduce the reader to key texts in both fields. The second section is a review of conceptualizations of teacher beliefs and epistemologies, in which we highlight the ways in which beliefs and epistemologies are characterized. The third section explores relationships between teachers’ beliefs/epistemologies and teaching and learning practices. Measuring teachers’ beliefs and epistemologies is the focus of the fourth section, which explicates different ways in which beliefs and epistemologies have been studied. Finally, in the fifth section, we examine research and theorization about the ways in which teachers’ beliefs and epistemologies might undergo change.