We shall present theoretical and experimental results that show that the very same interaction, namely the Fröhlich coupling between electrons and optical phonons, lead to a weak coupling in 3D/2D electron states while electrons in III-V or II-VI Quantum Dots enter a strong coupling with the LO phonons and form polarons. A particular attention has been paid to measure and interpret the energy relaxation in QD's where the S-P transition energy is far below the LO phonons. Pump and probe experiments and modeling of the energy relaxation from the upper states point out that the dominant mechanism is the coupling of the polarons to the 2 phonons reservoir due to anharmonicity down to THz energies. The situation of QW's in a strong magnetic field is more involved since the electron - phonon interaction is neither weak nor are the magneto - polarons long lived enough to support the strong coupling description. This work has been done in collaboration with the Sheffield group and the Dresden free electron laser group.