  Using mock observations of numerical simulations, we investigate the
completeness and efficiency of searches for galaxy clusters in weak lensing
surveys. While it is possible to search for high mass objects directly as
density enhancements using weak lensing, we find that line-of-sight projection
effects can be quite serious. For the search methods that we consider, to
obtain high completeness requires acceptance of a very high false-positive
rate. The false positive rate can be reduced only by significantly degrading
the completeness. Both completeness and efficiency are dependent upon the
filter used in the search and the desired mass threshold, emphasizing that a
measurement of the 3D mass function from gravitational lensing is affected by a
number of biases which mix `cosmological' and observational issues.
