This mixed-methods study applied a decision-making theoretical framework to an investigation of teacher grading in a large urban school district in California. A sample of 251 high school teachers of core subjects were surveyed, and 15 teachers participated in four focus group interviews in order provide data on the influences, rationale, and practices of the grading decision-making process. Findings identified three important influences on grading decisions and two components of teachers' rationales for grading. The findings also indicated that teachers' grading practices included factors such as students' effort and ability level as well as their achievement. The discussion, presented in the context of relevant research, describes multiple sources of subjectivity that contributed to inaccurate and inflated grades.