At first sight, the concept of web personalization looks deceivingly simple. A web personalization system is a software component that collects information on visitors to a web site and leverages this knowledge to deliver them the right content, tailoring presentation to the user's needs. All over the world, web designers and web content managers rely on web personalization solutions to improve the effectiveness and - ability of their web-based applications. Still, the scientific foundation of web personalization remains a controversial issue. Practitioners know very well that when properly implemented, personalization del- ers a much better user experience; but when it is poorly implemented, personalization may backfire and even distract the user's attention away from some useful (and co- ly-to-develop) enriched content. In other words, tailoring content, and varying it routinely, may make a site more attractive; but an unstable site look can have a negative impact on the overall m- sage. Everybody seems to agree that this is a real danger; but there are specific qu- tions that are much harder to answer convincingly.