<c:out/> unknown tag

You're apparently developing with a servlet container which does not support JSTL out the box, such as Tomcat. In that case, you need to download jstl-1.2.jar and drop in /WEB-INF/lib folder of your webapp. No other changes are necessary, also not extracting the JAR file and/or littering the /WEB-INF folder with loose TLD files as some poor online tutorials suggest.

After having dropped the JAR file in the classpath (the /WEB-INF/lib folder is part of the webapp's runtime classpath), you should be able to reference the JSTL core taglib by putting the following line in top of your JSP as per its documentation:

<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>

JSTL 1.2 requires a minimum of Servlet 2.4 declaration in web.xml. So make sure that your web.xml has the proper root declaration, preferably the highest supported version as supported by your servlet container (Tomcat 7 is Servlet 3.0, Tomcat 6 is Servlet 2.5 and Tomcat 5.5 is Servlet 2.4).

See also:

• Our JSTL tag wiki page (you can get to this page by putting your mouse above the [jstl] tag which you put on the question yourself and clicking the info link on the popbox)