请高手们帮忙看一下， 这个是什么错!
javax.servlet.ServletException: Servlet execution threw an exception

java.lang.NoClassDefFoundError: org/hibernate/cfg/Configuration
com.yourcompany.struts.action.LoginAction.execute(LoginAction.java:46)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:419)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)
org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
javax.servlet.http.HttpServlet.service(HttpServlet.java:763)
javax.servlet.http.HttpServlet.service(HttpServlet.java:856)

单独运行Struts的时候，不报错，单独运行Hibernate的时候也不报错，它们一起运行的时候就有错了！
怎么解决！