In this position paper we analyze the problem of Web Applications personalization from a design point of view. We focus on which design constructs are necessary to achieve modular and evolvable personalized Web Applications. We claim that personalization involves different concerns (e.g. rules, profiles, etc.) that should be clearly identified and decoupled; we show how to add personalized behaviors to existing applications in a non-intrusive way, by using reflective mechanisms found in most object-oriented languages. We first introduce our approach to Web Applications modeling that separates conceptual from navigation and user interface design; we next introduce personalization patterns and briefly show how they can guide the designer towards his objective. We finally present our approach and some ongoing research directions related with the design of an object-oriented framework for Web Applications personalization.