  Despite consistent progress in numerical simulations, the observable
properties of galaxy clusters are difficult to predict ab initio. It is
therefore important to compare both theoretical and observational results to a
direct measure of the cluster mass. This can be done by measuring the
gravitational lensing effects caused by the bending of light by the cluster
mass distribution. In this review we discuss how this phenomenon can be used to
determine cluster masses and study the mass distribution itself. As sample
sizes increase, the accuracy of the weak lensing mass estimates needs to
improve accordingly. We discuss the main practical aspects of these
measurements. We review a number of applications and highlight some recent
results.
