The Indonesian Throughflow (ITF) links two oceans and in so doing provides a pathway for modifying the stratification within each of these oceans as well as sea-air fluxes that impact on such climate phenomena as ENSO and the Asian Monsoon. While a number of measurement programs have recently been undertaken in the Indonesian region, a serious shortcoming is their lack of temporal coherence: the data cover different time periods and depths in the different passages of the complex pathways linking the Pacific and Indian Oceans. This has lead to ambiguity of the mean and variable nature of the ITF, and of the transformation of the thermohaline and transport profiles within the interior seas. The INSTANT program, involving contributions of 6 countries (USA, France, Netherlands, Australia, Indonesia, Japan), will finally measure in a coordinated fashion the ITF in the key throughflow passages (Makassar Strait, Lifamatola Passage, Lombok Strait, Ombai Strait and Timor Passage) simultaneously. The measurements span a 3 year period beginning in August 2003. The INSTANT objectives are: 1) To determine the full depth velocity and property structure of the Throughflow and its associated heat and freshwater flux; 2) To resolve the annual, seasonal, and intraseasonal characteristics of the ITF transport and property flux; 3) To investigate the storage and modification of the ITF waters within the internal Indonesian seas, from their Pacific source characteristics to the Indonesian Throughflow water exported into the Indian Ocean; and 4) To contribute to the design of a cost-effective, long-term monitoring strategy for the ITF. The intellectual merit and broader impact of INSTANT is clear: a quantitative view of the ITF will be gained and this will establish the basis for a long term "proxy" monitoring system, as required to capture the low frequency ITF variability. The ITF waters are known to play an integral part in the global thermohaline circulation and in the inter-basin transfer of climate signals and their anomalies. The benefit to the broader oceanographic community will be to finally allow the ITF to be faithfully simulated within ocean and climate models. In addition, INSTANT will involve education for a graduate student, and will offer research opportunities and training to undergraduates in both Indonesia and the US.