The main search tools for finding books, articles, databases, archival collections, and more at Yale (and beyond) can all be found on the "Find, Request, and Use" page of the library's website. Here are a few quick refreshers, but be sure to visit the page for a full overview:

The Yale Library has seven different special collections units, all of which hold archival collections in addition to rare and unique books and other published and unpublished materials that can serve as primary sources for your research projects. Special collections materials are described in Orbis/Quicksearch and archival collections more specifically in Archives at Yale. Additionally, the following online guides will assist you in using Yale's special collections and finding primary sources for your research project.

• The Guide to Using Special Collections at Yale University is a great introduction to finding and using special collections and archives held at Yale for your research projects. 
• The Primary Sources at Yale website is a great resource if you're not sure what types of materials might serve as primary sources for your research project. The section on database search strategies and using Advanced Search features is especially useful and will increase the power and focus of your database searches! Be aware that primary sources can be found in many library locations and resources other than the special collections units.