Within the next few years, traditional online public access catalogs will be replaced by more robust and interconnected discovery layers that can serve as primary public interfaces to simultaneously search many separate collections of resources. Librarians have envisioned this type of discovery tool since the 1980s, and research shows that discovery layer functionality and features have already resolved, or will soon remedy, many of the known issues with the traditional online public access catalog. The authors of this article review the literature on discovery, focusing on the evolution from the traditional online public access catalog to newer discovery interfaces, and summarize what has been published regarding the selection and evaluation of these new tools. Based on this review, emerging trends in the acquisition of discovery layers are described, including: the amount of time libraries devote to selection and evaluation, the staff involved and their areas of expertise, which discovery tools were selected to review, the evaluation methods used, and the features and functionality evaluated. The authors conclude with a discussion of the opportunities and concerns for libraries and their patrons related to discovery efforts, and provide direction for future research. While discovery layers are still developing, they show great promise in increasing the use and usability of library resources. (Contains 2 tables.)