The relationship between affect and achievement in science was examined in two stages. First, a model was developed and tested for science-related affect, the complex of students' attitudes toward, interests in, and perceptions about science at school. The LISREL approach to path analysis was used to demonstrate the fit of the model to data collected from grade 8 students in two different schools on two different occasions. In the second stage of the research, multiple linear regression was used to examine the direction of the relationship between science related affect and achievement and to apportion variance common between previous and subsequent achievement and the components of science-related affect. It was found that affect is related more strongly to previous than subsequent achievement and that much of the common variance can be attributed to students' perceptions of their competence in science.