Having demonstrated that the optimal incentive contract in a dynamic principal-agent problem will be linear in certain environments, Holmstrom and Milgrom then used linear contracts to explore in more detail what happens when agents allocate their efforts or attention across multiple tasks. Prior to 1991, models had generally considered effort on a single task. To reward performance on a single task, a principal can either reward performance (or some measure of it) or change the agent's opportunity cost of performing that task. This second strategy is key to understanding what happens when an agent has more than one task to which he can allocate effort, because increasing the reward on one task will generally alter the agent's opportunity cost of allocating effort to other tasks, increasing it when the tasks are substitutes for the agent and decreasing it when the tasks are complements. Holmstrom and Milgrom's (1991) paper demonstrates that when tasks are substitutes for the agent and it is difficult to measure performance on one of them, it may be optimal to have low-powered incentives, or even no incentives, on all tasks, even if some can be easily measured. They also demonstrated that the difficulties of providing incentives on multiple tasks have implications for the design of jobs. For instance, it may be better to split conflicting tasks between agents or to vary the intensity of monitoring and communication. Finally, in their 1994 paper, Holmstrom and Milgrom broadened the scope of their analysis to include not only performance-related pay but also other management choices that affect agents' incentives, such as choices about how much discretion to give agents and about whether or not agents own the assets with which they work. This paper stressed the interactions (the "complementarities") between these different choices, showing that the optimal choices for the principal will often vary together as the contracting environment changes. Holmstrom recounted the impact of this work at the Nemmers Conference in Honor of Paul Milgrom.