This documentation is for XNAT versions 1.6.0 - 1.6.5. You can find the latest documentation for XNAT 1.7 at https://xnat.org/documentation
XNAT is built to run within a Tomcat webserver and uses a PostgreSQL database to store data. To install XNAT, you will need to first install its prerequisites (Java, PostgreSQL, and Tomcat). After successfully installing the prerequisites, you can download the XNAT site builder which will build an XNAT web application and deploy it to Tomcat. If you are experienced with XNAT, you can also download developmental versions of XNAT using the Build XNAT from Source Repository instructions.
These applications should be installed and tested prior to installing XNAT:
Make sure to install the JDK and not the JRE.
Oracle very much wants you to download Java 8. Java 8 is acceptable for clients, i.e. if you're running XNAT applets in your browser. XNAT 1.6.4 does not work with Java 8! Make sure you download the latest version of the Java 7 JDK. We are working on migrating XNAT to work with Java 8, but there's no timetable for delivery.
As of the 1.6.5 release, XNAT is not compatible with Apache Tomcat 8. Please make sure that you download the latest version of Tomcat 7, but there are a number of known issues with XNAT running on Tomcat 8.