In this manual we present JET (Java Estimate Tsunami) ­ the Decision Support System for managing earthquake location, tsunami alert messages, and tsunami waveforms analysis in use at the Centro Allerta Tsunami (CAT­INGV) of the Istituto Nazionale di Geofisica e Vulcanologia (INGV) in Rome, Italy. INGV has the responsibility for the operations of the Italian National Tsunami Warning Centre (It­ NTWC) in the framework of the NEAMTWS and as such is acting as CTSP through the CAT­INGV since October 2014. CAT­INGV has been put into service within the preexisting National Seismic Monitoring Centre at the INGV headquarters in Rome. JET has been developed by CAT­INGV to carry out the fundamental decision support operations of the the real time analysis of seismic signals, for the determination of the alert level, and of sea level gauges signals, for the confirmation/cancellation/ending of the tsunami alerts, and to send the messages to the NEAM subscribers, and to Italian Department of Civil Protection (DPC). DPC is responsible for disseminating warning messages to local authorities and population in Italy.