While virtually all cosmological observations support the standard, 
Lambda-CDM cosmological model, galaxy clusters pose yet unanswered puzzles. 
Among them and related with each other are the arc-statistics problem, the 
overconcentration problem and the problem of large Einstein radii. All of them 
refer to the unexpectedly strong gravitational-lensing effects produced by 
massive galaxy clusters. It appears as if such clusters were much more 
efficient lenses than theory can explain. The talk will review the status of 
the field and discuss possible solutions.