The J2EE platform is a Multi-Tiered System. A Tier is a logical or functional partitioning of a system. There are two types : 1. 2-Tier(Client/Server) 2. 3-Tier(or N-Tier) 2-Tier : - When the developers are not disciplined, The display logic, business logic and database logic are muddled up and/or duplicated in a 2-tier client server system. 3-Tier : The advantages of the Multi-Tier architecture are : - Forced separation of user interface logic and business logic. - Business logic sits on small number of centralized machines (may be just one). - Easy to maintain, to manage, to scale, loosely coupled etc.