It's important to note that it's not a single theory, but actually two theories:

* Special Relativity: Published in 1905, it deals with the relationship between space and time.

* General Relativity: Published in 1915, it extends special relativity to include gravity.

While Einstein is credited with developing the theory, he did build on the work of other physicists, including:

* Galileo Galilei: His work on motion laid the foundation for special relativity.

* Isaac Newton: His law of universal gravitation was a key starting point for general relativity.

* Hendrik Lorentz: He developed the Lorentz transformations, which are essential to special relativity.

So, while Einstein is the name most associated with relativity, it's important to acknowledge the contributions of other scientists who paved the way for his groundbreaking work.