During the past decade, lanthanide doped alkali metal rare earth fluorides have been intensively studied due to their unique properties. Also, nanoparticles of these materials have gained much importance because NaYF4, NaGdF4, NaLuF4 and LiYF4 nanocrystals doped with Yb/Er or Yb/Tm display efficient upconversion emission. The synthesis of NaREF4 nanocrystals (RE = rare earths) has therefore attracted many researchers worldwide. While a large number of procedures is already available for the synthesis of NaREF4 particles of the heavier rare earth ions and for NaYF4, only a very limited number of methods exists for nanocrystals of the lighter rare earth ions, RE = La, Ce, Pr and Nd. In this work, a synthesis method was therefore developed to produce monodisperse sub-10 nm sodium rare earth fluoride nanocrystals of the lighter rare earths, NaREF4 (RE = La, Ce, Pr and Nd) from single-source precursors. Based on this method, the Ostwald ripening, the size focusing and the stability of these nanocrystals were studied; also, the versatility of the method was demonstrated by developing doped nanocrystals of these particles. The procedures developed in this thesis not only allow to prepare the hexagonal β-phase of these materials but also the meta-stable cubic α-phase. The latter is found to be very sensitive to decomposition. This decomposition affects also the synthesis of NaREF4 particles of the hexagonal β-phase where particles of the cubic α-phase form an intermediate product. This thesis also shows that highly sodium deficient α-NaYF4 nanocrystals with a size of less than 10 nm can be prepared which contain much less sodium than the bulk material. These nanocrystals were used as precursor to produce monodisperse sub-10 nm lithium rare earth fluoride (LiREF4, RE = Y, Gd, Lu) nanocrystals. Using this new approach also LiYF4:Yb,Er/LiYF4 core/shell upconversion nanocrystals were prepared, displaying a 35 times enhancement in luminescence intensity compared to the corresponding core particles. Further, this approach provided new insights on intermediate phases formed during the synthesis. Our studies confirmed, for instance, that LiREF4 nanocrystals were formed through an ‘available intermediate phase’, that is, a phase having a composition which is determined by the available cations in the reaction mixture.