A. The most popular limits

The most popular limits are shown just below the search box:

So you can limit right away, when you type your search term. E.g., if you were researching tooth abrasion and weren't interested in animal studies, you could limit your search to "Human":

This would eliminate about 300 of the articles about Tooth Abrasion:

Another popular limit is Publication Year. Let's say you're researching fluoridation, and you only want articles published between 2002 and 2004. You use the drop-down menus for Publication Year:

So, rather than getting 4,000 articles, you get just 109.

Another useful limit is "Review Articles". Let's say you're researching tooth remineralization, and you've decided to limit your search to review articles (i.e., articles which summarize past research on a particular subject and describe the current state of knowledge in the area):

You would find 98 review articles with the Subject Heading Tooth Remineralization:

You can limit in multiple ways at once. E.g., you could limit your search to review articles that are in English and were published between 2000 and 2004:

You'd get 22 articles:

As you can see, when there are a lot of articles on your topic, limiting can be a great way of zeroing in on what you need.