Classroom assistants and one-to-one assistants are an important part of the staffing structure of many autism support classrooms. Limited studies, however, have examined how one-to-one assistants spend their time in the classroom. The purpose of this article was to examine the percentage of time one-to-one assistants were engaged in instruction or support of students with autism and to determine the factors associated with their engagement. Direct observations were conducted in 46 autism support classrooms. Teachers and classroom assistants were engaged in instruction or support 98% and 91% of the time, respectively. One-to-one assistants were engaged in instruction or support 57% of the time. Classroom assistants' and one-to-one assistants' engagement was significantly correlated. The low rate of one-to-one assistants' engagement suggests an inefficient use of an important resource.