Form factors of hadronic systems in various forms of relativistic quantum mechanics

The form factor of hadronic systems in various forms of relativistic quantum mechanics is considered. Motivated by the agreement of the nucleon ``point-form'' results with experiment, results for a toy model corresponding to the simplest Feynman diagram are first presented. These ones include the results for this diagram, which plays the role of an experiment, for the front-form and instant-form in standard kinematics (q^+=0 and Breit frame), but also in unconventional kinematics and finally a Dirac's point-form inspired approach. Results for an earlier ``point-form'' approach are reminded. Results are also presented for the pion charge form factor. Conclusions as for the efficiency of various approaches are given.