We have known since 1986 that binary black hole mergers are gravitational-wave 
standard sirens, allowing us to measure luminosity distances. Advanced LIGO and 
VIRGO are likely to make the first uses of this for measuring the Hubble constant, 
but LISA has the potential to turn this into a tool for measuring the dark-energy 
equation of state and possibly even its evolution. As we understand more and more 
about the signals from these systems our estimates of the accuracy of LISA's position 
measurements have improved dramatically, and with it the likelihood that we will be 
able to identify the host clusters of many of the mergers. The remaining challenge 
will be to control the distortions in apparent gravitational luminosity induced by 
weak lensing as the signals travel over cosmological distances to us. If this can be 
controlled, the potential returns for cosmology and fundamental physics are very exciting.