This study unpacks the effective Exceptional Coaching for Early Language and Literacy ("ExCELL") professional development intervention to understand how the language and literacy coaching that teachers receive is linked to changes in their knowledge and then how this knowledge is related to classroom practice. In total, 17 lead teachers participated in the intervention and 10 lead teachers participated in the control condition; all were recruited from 3 Head Start centers in a major urban area in the northeast. Results showed that teachers varied in their knowledge about language and literacy--as measured by the "ExCELL" Teacher Knowledge Measure--at the start of the intervention, and teachers with more years of education had higher average levels of knowledge. The "ExCELL" intervention was linked to significant increases in teacher knowledge, particularly in the areas of writing and book reading. Finally, higher levels of teacher knowledge were related to stronger classroom instructional quality, albeit marginally. (Contains 5 tables.)