To aid designers of digital library interfaces and web sites in creating comprehensible, predictable and controllable environments for their users, we define and discuss the benefits of previews and overviews as visual information representations. Previews and overviews are graphic or textual representations of information abstracted from primary information objects. They serve as surrogates for those objects. When utilized properly, previews and overviews allow users to rapidly discriminate objects of interest from those not of interest, and to more fully understand the scope and nature of large collections of information resources. We provide a more complete definition of previews and overviews, and discuss system parameters and aspects of primary information objects relevant to designing effective preview and overviews. Finally, we present examples that illustrate the use of previews and overviews and offer suggestions for designers. (Also cross-referenced as UMIACS-TR-97-73)