Apert, Pfeiffer and Crouzon syndromes are congenital craniosynostosis syndromes caused by mutations that perturb the level of fibroblast growth factor receptor (FGFR) signaling. The cellular and molecular impact of these mutations have been studied in vitro and in animal models in vivo. Here, I highlight the complexity of the FGF/FGFR signaling system and review the candidate modifiers responsible for regulating the levels of FGF/FGFR signaling in tissues. I also review what we have learned from the phenotypic analysis of mice that model these craniosynostosis syndromes and discuss some in vivo strategies for further understanding as well as alleviating the associated craniofacial defects.