We examined the relations of teacher knowledge (n = 42 first-grade teachers), explicit decoding instruction provided, and students' (n = 437) word-reading gains. Results revealed an interaction between teacher knowledge and observed decoding instruction: For students of more knowledgeable teachers, more time in explicit instruction predicted stronger word-reading gains. For students of less knowledgeable teachers, more time in explicit instruction was associated with weaker skill gains. Findings highlight the importance of teachers' specialized body of knowledge about reading as it informs effective instruction.