This work presents our research into visualization for digital repository search interfaces, motivated by the prevalence of existing hierarchical data structures and the general lack of contextualization present in existing systems. We develop the ResultMap concept, a treemap-based visualization that we have applied to keyword search engine and faceted classification data environments, and present the results of their empirical evaluation, which show limited objective and subjective benefits for some users and no detrimental effects in any cases. We organize this work as follows: Chapter 1 provides an introduction to our problem area, motivates our general approach of leveraging hierarchical structure (via ResultMaps) for context, and proposes a thesis statement and corresponding research questions. Chapter 2 discusses related work, and includes a survey and design characterization of faceted navigation tools. Chapter 3 defines the key visual and interactive features of the ResultMap concept and justifies their basic design. Chapter 4 presents our implementation and evaluation of ResultMaps applied to digital library search engine result pages (SERPs). Chapter 5 consists of two major portions: a presentation of formal data and query models for faceted environments, and our implementation and evaluation of ResultMaps in a faceted UI context. In Chapter 6 we conclude--based on our results from Chapter 4 and Chapter 5--with a set of principles for designing both visual search interfaces themselves and designing their evaluation. We finish with suggestions for future research in this area.