The spin excitation spectrum in the 2D square lattice Heisenberg antiferromagnet CFTD has been investigated using time-of-flight neutron scattering. The magnon dispersion and intensities have been determined as a function of wavevector Q in the full Brillouin zone. Both quantities are found to deviate significantly from the predictions of linear spin wave theory for wavevectors in the vicinity of Q = (pi, 0), whereas good agreement is found near Q = (pi/2, pi/2).