Raffaele, Hashtable is synchronized already, and so you will not have any problem. But answering your question, drools does not start any inner thread. It will simply use the thread calling its methods, so the default java synchronization mechanisms apply. []s Edson 2007/8/30, Raffaele Viola <raffovi(a)gmail.com&gt;: -- Edson Tirelli Software Engineer - JBoss Rules Core Developer Office: +55 11 3529-6000 Mobile: +55 11 9287-5646 JBoss, a division of Red Hat @ www.jboss.com