  Fluctuating properties of the atmosphere, and in particular its water vapour
content, give rise to phase fluctuations of astronomical signals which, if
uncorrected, lead to rapid deterioration of performance of (sub)-mm
interferometers on long baselines. The Atacama Large Millimetre/submillimeter
Array (ALMA) uses a 183 GHz Water Vapour Radiometer (WVR) system to help
correct these fluctuations and provide much improved performance on long
baselines and at high frequencies. Here we describe the design of the overall
ALMA WVR system, the choice of design parameters and the data processing
strategy. We also present results of initial tests that demonstrate both the
large improvement in phase stability that can be achieved and the very low
contribution to phase noise from the WVRs. Finally, we describe briefly the
main limiting factors to the accuracy of phase correction seen in these initial
tests; namely, the degrading influence of cloud and the residual phase
fluctuations that are most likely to be due to variations in the density of the
dry component of the air
