This dissertation examines four college professors’ approaches to writing instruction in the disciplines of history and engineering. An investigation of writing instruction in two disparate disciplinary contexts contributes to our understanding of how instructors approach writing instruction in the disciplines, and which factors encourage and inhibit writing instruction. This study proposes and assesses the validity of a guiding conceptual framework, which posits that the primary factors influencing faculty’s approaches to writing instruction are academic biography, disciplinary identity, and educational ideology. The study employs a qualitative case study methodology, and data sources include in-depth interviews, field observations, and analysis of documents such as syllabi and writing prompts. This dissertation is founded on a premise that the instructor is an under-studied but essential player in the Writing in the Disciplines movement. The study reveals more about the nature of discipline-based writing instruction, and proposes a conceptual framework for future research on instructional approaches to disciplinary writing.