Philosophiae Doctor - PhD / Next-generation galaxy surveys will usher in a new era of high precision cosmology. They will increasingly rely on the galaxy bispectrum to provide improved constraints on the key parameters of a cosmological model to percent level or even beyond. Hereby, it is imperative to understand the theory of the galaxy bispectrum to at least the same level of precision. By this, we mean to include all the general relativistic projection effects arising from observing on the past lightcone, which still remains a theoretical challenge. This is because unlike the galaxy power spectrum, the galaxy bispectrum requires these lightcone corrections at second-order. For the rst time, this PhD project looks at all the local relativistic lightcone e ects in the galaxy bispectrum for a at Friedmann-Lemaitre-Robertson-Walker Universe, giving full details on the second-order scalars, vectors and tensors. These lightcone effects are mostly Doppler and gravitational potential contributions. The vector and tensor modes are induced at second order by scalars. We focus on the squeezed shapes for the monopole of the galaxy bispectrum because non-Gaussianity of the local form shows high signatures for these triangular con gurations. In the exact squeezed limit, the contributions from the vectors and tensors vanish. These relativistic projection effects, if not included in the analysis of observations, can be mistaken for primordial non-Gaussianity. For future surveys which will probe equality scales and beyond, all the relativistic corrections will need to be considered for an accurate measurement of primordial non-Gaussianity.