Albert Einstein studied physics. He is best known for his groundbreaking work in theoretical physics, particularly his theory of relativity (special and general relativity). His work revolutionized our understanding of space, time, gravity, and the universe.

However, it's worth noting that Einstein's scientific interests were broad, and he also made significant contributions to:

* Mathematics: He used advanced mathematical tools to develop his theories.

* Cosmology: He made important contributions to our understanding of the structure and evolution of the universe.

* Quantum Mechanics: While he had reservations about some aspects of quantum mechanics, he made important contributions to its development.

Einstein's scientific contributions spanned a wide range, but he is primarily remembered for his revolutionary work in theoretical physics.