Abstract. The ISO Data Archive (IDA) and the XMM-Newton Science Archive (XSA) have been developed by the Science Operations and Data Systems Division of ESA in Villafranca, Spain. They are both built using the same flexible and modular 3-tier architecture: Data Products and Database, Business Logic, User Interface. This open architecture, together with Java and XML technology have helped in making the IDA and XSA inter-operable with other archives and applications. The various accesses from the IDA and the XSA to remote archives are described as well as the mechanism to directly access these ESA archives from remote archives Open and Flexible 3-Tier ArchitectureThe IDA and the XSA were both built-by a common team-using the open 3-tier architecture described in Figure 1. The main goal of this architecture is to separate the data from the presentation, which allows a more modular and flexible development.As the data volume is not that big, data are saved on magnetic disks for fast access as a normal UNIX file system. From the data products, metadata is extracted and put in a Relational Data Base, SYBASE. Note that the data ingestion from the data producer and the metadata extraction are separate processes to allow new metadata data extraction when user requirements evolve.The middle tier, also called the Business Logic, provides transparent access to the data products and to the metadata. This key layer has been developed in Java and XML and resides on the archive server.On the client side, several types of applications can be found. The standard IDA and XSA User Interface is a Java applet downloaded by the end user to access the archive content.Remote applications and other archives can also have access to the data and the metadata, bypassing the standard User Interface, by speaking to the Business Logic that will provide them with the required services via Java Server Pages.This architecture is especially powerful in the context of the worldwide Virtual Observatory initiatives where archives will all have to interoperate in a manner transparent to the end user. 47