In this paper we discuss the difficulties involved in the scheduling of applications on computational grids. We highlight two main sources of difficulties: firstly, the size of the grid rules out the possibility of using a centralized scheduler; secondly, since resources are managed by different parties, the scheduler must consider several different policies. Thus, we argue that scheduling applications on a grid require the orchestration of several schedulers, with possibly conflicting goals. We discuss how we have addressed this issue in the context of PAU?, a grid for Bag-of-Tasks applications (i.e. parallel applications whose tasks are independent) that we are currently deploying throughout Brazil.