This study examined teacher prompting during first grade reading intervention sessions to determine what conceptual understandings children must develop in order to understand the alphabetic principle, and what prompts teachers give to develop these understandings. Linguistic units (word, syllable, intra-syllable), phoneme, linguistic processes (detecting, linking, blending, segmenting), and word reading strategies (decoding, analogy, automatic) associated with these conceptual understandings were examined. Five categories of conceptual understanding emerged: Sounds, Letters, Words, Parts of Words, and Whole Words, with eight main conceptual understandings. The conceptual understandings referred to most frequently included: “Words are made up of sounds that are represented by letters in a sequential order (30%),” “Words are made up of parts. (19%),” “Letters represent sounds. (13%),” and “Words can be read and written automatically, without analysis (11%).” The data showed that teachers adjusted prompting in response to student reading development by shifting their focus on conceptual understandings, and by decreasing the number of prompts per lesson as students’ skills developed. Prompts addressed all linguistic units, focusing most frequently on phonemes. All linguistic processes were addressed, however teacher emphasis shifted as students’ abilities developed. Attention to word reading strategies changed as students developed, although decoding was the predominate strategy throughout lessons. Teachers used questioning, modeling, guiding and affirming useful behavior, rather than just telling, in order to develop independent readers. Teacher prompting is the enacted reading curriculum. The findings from this study support the conclusion that expert teachers are the solution to effective reading instruction, and teacher expertise is dependent on effective and ongoing professional development.