Overviews of three studies which focused on achievement as related to instructional time are presented. The first study investigated whether or not time spent in science with fourth and fifth graders (N=86) detracted from learning in more basic areas. The second study examined reading and mathematics achievement in grades 1 and 3 as related to initial performance and to four classroom processes. These processes were represented by four constructs: opportunity, motivators, instructional events, and structure. Opportunity consisted of two variables, time and curriculum overlap, with time being estimated by attendance, allocation, task rate, enrollment, and transfers. The third study investigated the nature of reading difficulties in learning disabled (LD) classes, type of student activities which lead to greatest improvement in reading test performance, and what types of instructional situations generate these student activities. Among the basic findings which have emerged from these studies are those indicating that time is overlapping and not mutually exclusive within a student or teacher, even when the focus is primarily on one type of activity; that time on task is not the same as time on the right task; and that allocated time may not be the upper bound for engaged time. (JN)