We consistently quantize a class of relativistic nonlocal field equati
ons characterized by a nonlocal kinetic term in the Lagrangian. We sol
ve the classical nonlocal equations of motion for a scalar field and e
valuate the on-shell Hamiltonian. The quantization is realized by impo
sing Heisenberg's equation, which leads to the commutator algebra obey
ed by the Fourier components of the field. We show that the field oper
ator carries, in general, a reducible representation of the Poincare g
roup. We also consider the Gupta-Bleuler quantization of a nonlocal ga
uge theory and analyze the propagators and the physical modes of the g
auge field.