The authors developed and validated a measure of teachers' motivation toward specific work tasks: The Work Tasks Motivation Scale for Teachers (WTMST). The WTMST is designed to assess five motivational constructs toward six work tasks (e.g., class preparation, teaching). The authors conducted a preliminary (n = 42) and a main study among elementary and high school teachers (n = 609) to develop and validate the scale. Overall, results from the main study reveal that the WTMST is composed of 30 reliable and valid factors reflecting five types of motivation among six work tasks carried out by teachers. Results based on an extension of the multitrait-multimethod approach provide very good support for assessing teachers' motivation toward various work tasks. Support for the invariance of the WTMST over gender and teaching levels was also obtained. Results are discussed in light of self-determination theory and the multitask perspective. (Contains 4 tables.)