  The gravitational waveform (GWF) generated by inspiralling compact binaries
moving in quasi-circular orbits is computed at the third post-Newtonian (3PN)
approximation to general relativity. Our motivation is two-fold: (i) To provide
accurate templates for the data analysis of gravitational wave inspiral signals
in laser interferometric detectors; (ii) To provide the associated
spin-weighted spherical harmonic decomposition to facilitate comparison and
match of the high post-Newtonian prediction for the inspiral waveform to the
numerically-generated waveforms for the merger and ringdown. This extension of
the GWF by half a PN order (with respect to previous work at 2.5PN order) is
based on the algorithm of the multipolar post-Minkowskian formalism, and
mandates the computation of the relations between the radiative, canonical and
source multipole moments for general sources at 3PN order. We also obtain the
3PN extension of the source multipole moments in the case of compact binaries,
and compute the contributions of hereditary terms (tails, tails-of-tails and
memory integrals) up to 3PN order. The end results are given for both the
complete plus and cross polarizations and the separate spin-weighted spherical
harmonic modes.
