In the 1860s James Maxwell showed that the electrical and magnetic forces were just different aspects of one unified theory - electromagnetism.
In the 1960s, Sheldon Glashow, Abdus Salam and Steven Weinberg unified the electromagnetic and weak interactions, producing the 'electroweak' interaction.
Physicists are now searching for a theory that unifies the electroweak and strong interactions. Such theories are known as grand unified theories (GUTs).
The ultimate goal is a theory that unifies all four forces, including gravity (for which no quantum theory as yet exists). Such theories are called theories of everything (TOEs).