New ICT technologies are changing library catalogs as global sources of knowledge both from the information infrastructure aspect as well as from the perspective of users’; expectations on data exploration and retrieval. New types of materials, content access as well as the availability of a vast amount of data have led to the development of discovery tools that allow a unified search of multiple sources that contain different data structures. The issue of finding relevant and valuable information among large volumes of data is a crucial one. One of the answers is the integration of the subject heading systems and thesauri into a centralized index. This paper will present the advantages of searching and retrieving information as well as relevancy ranking by utilizing the discovery tool with a central index. Furthermore, the advantages of using central subject indexes that integrate various types of subject indexes is discussed. A theoretical framework on the developmental and professional foundations of discovery tools and subject indexes is provided. From a more practical aspect, processes of ranking by relevance and automatic subject indexing will be presented as central technological innovations that enabled the development of discovery tools. The benefits of integrating subject indexes will be illustrated using examples from EBSCO’s EDS system. A comparative qualitative analysis of the two discovery tools will include EBSCO’s Discovery Service EDS and ProQuest’s Summon. The analysis will provide a comprehensive overview of the ways in which libraries are taking a step towards a new generation user, who shape their behavior based on the experience of using internet search engines.