  We show that diversity, in the form of quenched noise, can have a
constructive effect in the dynamics of extended systems. We first consider a
bistable $\phi^4$ model composed by many coupled units and show that the global
response to an external periodic forcing is enhanced under the presence of the
right amount of diversity (measured as the dispersion in one of the parameters
defining the model). As a second example, we consider a system of
active-rotators and show that while they are at rest in the homogeneous case,
the disorder introduced by the diversity suffices to trigger the appearance of
common firings or pulses. Both effects require very simple ingredients and we
expect the results presented here to be of interest in similar models.
