A worldwide interest during the past thirty years has been witnessed in the development of colloidal semiconductor nanocrystals (NCs). With significant efforts on the control of the size and size distribution, NC synthesis has been performed as an empirical art, lacking of fundamental understanding of the pre-nucleation stage also called the induction period. Colloidal semiconductor quantum dots (QDs) and magic sized clusters (MSCs) have been observed to evolve in a reaction together, sometimes. In this presentation, I will present our systematic study on their formation pathways. Based on our experimental data, we propose that there are two individual pathways in the prenucleation stage of the formation of QDs. The LaMer model of the classical nucleation theory (CNT) describes one pathway and the self-assembly of the cation and anion precursors is the other pathway which results in the formation of MSCs. We believe that our exploration of the prenucleation stage moves one step forward towards mechanism-enabled design and synthesis of NCs.