In the March/April issue of IEEE Internet Computing, I wrote about several Web Services specifications that define facilities and approaches required to support notification-based and event-driven systems. I had not intended to write a two-part column series. However, just after I finished that column, a group of companies, including Hewlett-Packard and IBM,published their WS-Notification specifications. Since then, several readers have asked why I didn't include these new specifications, so I've decided to cover them here.