The main variable in classroom performance is not students. It's not parents. It's not the principal or the board of education. It is the teacher. Numerous studies confirm that the most important factor contributing to student success is the effectiveness of instruction. And though school quality is largely determined by teacher quality, that single factor is surprisingly difficult to improve. The well-chronicled and often-exaggerated reasons for this include veteran teachers resisting change, younger instructors lacking experience, and improvement undermined by tenure's protection. However, another obstacle may hinder instructional growth with more certainty: Many educators view teaching as an art, in which best practices are elusive, hard to quantify, and difficult to express. This is unfortunate. What the very best teachers do is not some unknowable mystery. This article discusses the five habits of top-performing teachers.