Starting with early research on multiple source comprehension that primarily emerged from work in history, researchers have explored several types of instructional manipulations including altering the features of the inquiry task that is given (such as being asked to write a narrative or an argument); changing features of the task environment (such as the format of the source documents or features of the document set); and varying the instructional context (such as having students engage in a particular activity or training prior to engaging in a multiple-source inquiry task). These three broad categories continue to represent the main types of manipulations that have been studied as this literature has expanded to also include explorations into the comprehension of scientific phenomena from multiple-source inquiry tasks. This chapter provides an overview discussing the kinds of processes that are theoretically involved in multiple-source comprehension; articulates the challenges that readers face when they attempt to engage in multiple-source inquiry tasks; and summarizes the empirical research that has attempted to explore multiple source comprehension processes using manipulations of the inquiry task, the task environment, and the instructional context. [This paper was published in: J. L. G. Braasch, I. Bråten, & M. T. McCrudden (Eds.) "Handbook of Multiple Source Use" (p341-361). New York, NY: Routledge.]