Psychology of subject matter refers to the scientific study of learning and instruction within school subjects. The growing research literature on teaching and learning of school subjects represents one of educational psychology's most productive accomplishments of the past two decades. The purpose of this chapter is to examine representative advances in the psychology of subject matter, including how people learn to read words, comprehend printed passages, write compositions, solve arithmetic word problems, and understand how scientific systems work. The introduction provides a historical overview of how to promote transfer and is followed by reviews of representative research in learning and teaching of reading fluency, reading comprehension, writing, mathematics, and science.