In this work we review studies of the pressure dependence of the light emission from quantum structures and thick epitaxial layers of hexagonal and cubic group-III nitrides. In these semiconductors, an association of the band gap magnitude with the energy of light emission should be treated with caution. Usually, anomalously large Stokes shifts between light emission and absorption are observed, which can reach a value of 0.5-1.0 eV. Two effects are taken into account to explain this phenomenon: (i) in the hexagonal (wurtzite) nitride quantum structures large built-in electric fields caused by spontaneous and piezoelectric polarization dominate their optical properties; (ii) in hexagonal as well as in cubic nitrides In-distribution fluctuations lead to a strong Stokes shift effect, too. We show that both effects influence the pressure coefficient of the emitted light energy, dEE/dP, making it different from dEG/dP. In the final part we show that the built-in biaxial strain caused by a lattice mismatch can also lead to a reduction of dEE/dP.