In the past, seemingly different interaction fields (or "forces," in less precise terms) have been unified together. James Clerk Maxwell successfully unified electricity and magnetism into electromagnetism in the 1800s. The field of quantum electrodynamics, in the 1940s, successfully translated Maxwell's electromagnetism into the terms and mathematics of quantum mechanics.
In the 1960s & 1970s, physicists successfully unified the strong nuclear interaction and weak nuclear interactions together with quantum electrodynamics to form the Standard Model of quantum physics.
The current problem with a fully unified field theory is in finding a way to incorporate gravity (which is explained under Einstein's theory of general relativity) with the Standard Model that describes the quantum mechanical nature of the other three fundamental interactions. The curvature of spacetime that is fundamental to general relativity leads to difficulties in the quantum physics representations of the Standard Model.
Some specific theories that attempt to unify quantum physics with general relativity include: