Tamar Herzog's work centers on the relationship between Spain, Portugal, Portuguese and Spanish America and the ways by which Iberian societies changed as a result of their involvement in a colonial project.

Her first set (of four) books examined the working of colonial institutions in everyday situations. It included an analysis of the relationship between legal norms and social and political practices, and was mainly concerned with the way institutions and normative orders responded to changing circumstances, and to material and symbolic constraints. Her fifth book dealt with the way individuals negotiated being members of both local and kingdom communities, and with how immigrants became citizens, and citizens were transformed in outsiders. Her latest book examines the formation of the border between Spain and Portugal in both Europe and the Americas. Rather than a political, military or diplomatic history, it analyzes how boundaries were formed on the ground by neighbors and how the right to land and the use of territory were discussed, negotiated, obtained or denied.

A legal scholar and historian by training, her work engages with early modern European history, colonial Latin American history, imperial history, Atlantic history, and Legal history.