  We consider the problem of identifying n points in the plane using disks,
i.e., minimizing the number of disks so that each point is contained in a disk
and no two points are in exactly the same set of disks. This problem can be
seen as an instance of the test covering problem with geometric constraints on
the tests. We give tight lower and upper bounds on the number of disks needed
to identify any set of n points of the plane. In particular, we prove that if
there are no three colinear points nor four cocyclic points, then roughly n/3
disks are enough, improving the known bound of (n+1)/2 when we only require
that no three points are colinear.
  We also consider complexity issues when the radius of the disks is fixed,
proving that this problem is NP-complete. In contrast, we give a linear-time
algorithm computing the exact number of disks if the points are colinear.
