After deciding which database(s) you'd like to conduct a search in, find the Subjects tab underneath the search box fields. It will be next to Filters, Search options, and Publications.
If you are searching within multiple databases, there may be more than one controlled vocabulary list available for browsing under the Subjects tab. For example, if you are searching within both the PsycINFO and Academic Search Complete databases, both the APA Thesaurus of Psychological Index Terms and the ASC Subject Terms list will be available.
If you do not see a Subjects tab option, it is because the selected database(s) is/are not associated with any thesauri or classification rules.

Select which thesaurus or other controlled vocabulary list you'd like to browse from the drop down menu. The drop down menu will not appear if only one thesaurus is available.
Conduct a search by typing your keyword(s) in the "Search for terms in" box. Choose whether you'd like to see all thesaurus terms that begin with your chosen keyword or phrase OR if you'd like your results ranked by relevancy. Click the magnifying glass to conduct the search and browse the results.

Terms in the results list will be accompanied by check boxes to the left-hand side. Some terms may also have a check box in the "Explode" column; clicking an Explode box for a term will automatically select a group of terms that are associated with/are subcategories of that overarching term.
Check the boxes next to each term you'd like to add to your overall search query. If you've selected more than one term, you will also have the option to select a Boolean operator (AND, OR, or NOT) to connect them via the "Join terms using:" drop down menu. Click "Add to search" to add your selected terms to your overall search query.

Keywords added from the Subjects tab will form a search string with any search terms already entered into the main search box above. For example, if you've already typed "post-traumatic stress disorder" into the main search box, adding the term "adolescent development" from the APA Psychological Index Terms list from the Subjects tab will transform your overall search query into "post-traumatic stress disorder AND/OR/NOT adolescent development," depending upon which Boolean operator you choose to connect the terms.
Click "Search" to conduct a search query using your selected terms.