Quantum field theory treats fields as fundamental and particles as excitations of those fields. This viewpoint unifies wave-particle behavior with creation and annihilation processes that cannot be described in ordinary single-particle quantum mechanics.

In field theory, the action is built from a Lagrangian density $\mathcal{L}$ integrated over spacetime: $$S=\int \mathcal{L}\,d^4x.$$ The equations of motion follow from a field version of the Euler-Lagrange equations. Local interactions are encoded directly in $\mathcal{L}$.

Modern fundamental interactions are often described by gauge symmetries, where the theory is invariant under local transformations. Electromagnetism is a $U(1)$ gauge theory; the weak and strong interactions arise from non-Abelian gauge groups. Gauge symmetry restricts the possible interactions and introduces force-carrying fields.

The Standard Model combines quantum electrodynamics, the weak interaction, and quantum chromodynamics into a gauge-theoretic framework. Matter fields are organized into fermions, and interactions arise through gauge bosons and the Higgs field, which gives masses to some particles after spontaneous symmetry breaking.