I've had this a few weeks ago. You are probably starting Tomcat from somewhere other than the Solr home.
See "Simple Example Install" section of http://wiki.apache.org/solr/SolrTomcat There, tomcat is started from the Solr home by: ./apache-tomcat-5.5.20/bin/startup.sh If you do cd apache-tomcat-5.5.20/bin ./startup.sh you'll get this error. (I have no idea why the error is reported as NoClassDefFoundError, which is not what it is. Some improvement is needed in the error reporting, IMHO.) If you want to start Tomcat from other location, follow the instructions in the next subsection of this page. Hope this helps. -kuro
