The PubMed Advanced search page has been changed to provide a clearer, more intuitive way to create search strategies.   See descriptive figures here.

Terms entered in the Search Builder will automatically appear in the search box.  When additional terms are entered, they are added with the Boolean operator AND unless OR or NOT are selected from the pull-down menu.  The MeSH terms field now incorporates an auto complete feature.

Watch a You Tube tutorial of the new version of the Advanced Search page for a demonstration.