Tomcat 6.0 is the current focus of development. It builds upon the improvements made in Tomcat 5.5.x and implements the Servlet 2.5 and JSP 2.1 specifications. In addition to that, it includes the following improvements: Memory usage optimizations, Advanced IO capabilities, Refactored clustering.

Apache Tomcat 5.5 supports the same Servlet and JSP Specification versions as Apache Tomcat 5.0.x, there are significant changes in many areas under the hood, resulting in improved performance, stability, and total cost of ownership.

Improves on Apache Tomcat 4.1 in many ways, including: Performance optimizations and reduced garbage collection, Refactored application deployer, with an optional standalone deployer allowing validation and compilation of a web application before putting it in production, Complete server monitoring using JMX and the manager web application, Improved Taglibs handling, including advanced pooling and tag plugins, Embedding using JMX, Enhanced Security Manager support, Integrated session clustering.

Implements a new servlet container (called Catalina) that is based on completely new architecture. Apache Tomcat 4.1 is a refactoring of Apache Tomcat 4.0.x, and contains significant enhancements, including: JMX based administration features, JSP and Struts based administration web application, New Coyote connector (HTTP/1.1, AJP 1.3 and JNI support), Rewritten Jasper JSP page compiler, Custom Ant tasks to interact with the manager application directly from build.xml scripts.