Thanks to gravitational lensing magnifications, massive clusters act as natural telescopes that enable us to study faint and distant sources that would otherwise be inaccessible. In order to extract accurate and unbiased information from these magnified sources, it is crucial to reconstruct mass distributions of massive clusters that act as lenses as accurately as possible. I will present recent results of cluster mass modeling from strong gravitational lensing, including those from the innovative Hubble Frontier Fields project, and discuss how we can test and improve the accuracy of cluster mass models.