We describe the development of the Measure of Affect in Science and Technology (MAST), and study its usefulness for measuring science affect in middle school students via both classical and Rasch measurement perspectives. We then proceed to utilize the measurement structure of the MAST to understand how middle school students at varying levels of affect express their interest and attitudes toward science and technology and gender differences in how students express their affect. We found that affect in science and technology comprises a main dimension, science interest, and four peripheral dimensions: interest in careers in science and technology, attitudes toward science, and interest in attending science class. Of these, careers in science and technology carry the highest affective demand. While males showed higher levels of personal and situational interest in science, a greater interest in careers in science and technology was the biggest contributor to males' higher affect toward science and technology. We argue that whether the MAST is used as a measure of a single construct or multiple subconstructs depends upon specific research or evaluation goals; however, both uses of the MAST yield measures which produce valid inferences for student affect.