We demonstrate that the optical properties of GaInN/GaN/AlGaN quantum wells are governed to a large extent by the spontaneous and piezoelectric polarization fields arising from the strongly polar nature of wurtzite nitrides. The spontaneous emission from single quantum wells is strongly red-shifted and exhibits a very small oscillator strength, particularly for wide wells. In multiple quantum wells the electric fields lead to spatially highly indirect transitions between nearest and next nearest neighbor quantum wells. Screening of the fields due to injected carriers leads to a strong blue-shift of the emission. Internal fields due to spontaneous polarization are much more elusive than piezoelectric ones. By manipulating the surface coverage using an electron beam we demonstrate that such spontaneous fields in fact exist and that their direction is opposite to the piezoelectric fields. In this way, the emission of a nitride quantum well can be shifted by as much as 500 meV in a metastable manner. We discuss the impact of internal polarization fields on LED efficiency. We show that nonradiative recombination is subject to reduction by field effects in a similar manner as the radiative one.