Peculiar velocities are an important probe of the growth rate of mass density fluctuations in the Universe. Most previous studies have focussed exclusively on measuring peculiar velocities at intermediate ($0.2 < z < 1$) redshifts using statistical r
edshift-space distortions. Here we emphasize the power of peculiar velocities obtained directly from distance measurements at low redshift ($z lesssim 0.05$), and show that these data break the usual degeneracies in the Omega_{m,0} -- $sigma_{8,0}$ parameter space. Using only peculiar velocity data, we find $Omega_{m,0} = 0.259pm0.045$ and $sigma_{8,0} = 0.748pm0.035$. Fixing the amplitude of fluctuations at very high redshift using observations of the Cosmic Microwave Background (CMB), the same data can be used to constrain the growth index $gamma$, with the strongest constraints coming from peculiar velocity measurements in the nearby Universe. We find $gamma = 0.619pm 0.054$, consistent with LCDM. Current peculiar velocity data already strongly constrain modified gravity models, and will be a powerful test as data accumulate.
