Citation searching is a method for locating additional relevant publications relating to your field of research by selecting one relevant reference from a known source.
This type of searching allows you to go back in time and locate papers the authors have read to inform their article, and by discovering other articles that cite your original reference, thereby going forward in time.
The advantage of citation searching is that you can follow a paper trail of scholarly communication on a given topic over time, by going backward and forward from your original reference.
Web of Science - search for items that have cited published works with in the Web of Science database.
Scopus - this tutorial demonstrates how you can access the references which appear in the Scopus documents and the documents which cite the Scopus documents.
Google scholar - has links to 'cited by' articles and the option to 'search within articles' once you have searched for a known item.