Processes of classroom emotional transmission have been identified whereby the emotions expressed by an individual are induced in others, with particular attention paid to how this unfolds among teachers and their students. However, there is still much to be clarified about how teachers' and students' emotions transmit in the classroom, including the extent to which the teachers' observable emotional expressions are meaningful for students' learning experiences and the potential context-specificity of these associations. In the present study, we introduce the Teacher Affect Coding System (TACS), a new observational measure of teachers' emotional expressions and use data from this measure to describe the factor structure and nature of these emotional expressions as teachers instruct in mathematics and English language arts (ELA). We then use TACS data in combination with students' self-reports of their content area enjoyment, anxiety, and behavioral engagement to conduct an exploratory analysis of the associations among teachers' expressions and their students' learning-related emotions and behaviors in each content area. Participants included 65 fourth-grade teachers and 805 students recruited from a Southwestern U.S. state. Confirmatory factor analyses revealed four factors differentiating between positive and negative emotions in each content area. Descriptive analyses indicated that teachers displayed more positive emotions than negative, and that emotional expressions were fairly consistent across content areas. Path analyses revealed that teachers' negative emotional expressions in ELA were associated with reduced student ELA enjoyment and engagement, and negative emotional expressions in mathematics were associated with reduced student mathematics engagement. Findings speak to the importance of teacher training and professional learning opportunities focusing on emotional regulation skills among elementary teachers. [This is the online first version of an article published in "Contemporary Educational Psychology."]