We describe the basic physics of a Quantum Single Electron Transistor (QSET). The QSET is a quantum dot (QD) connected to two leads and a central electrode. The effect of the central electrode, electron-electron interaction, and a magnetic field on electron droplets in the QSET is studied using the Hartree-Fock approximation and exact diagonalization techniques. In a strong magnetic field, the central electrode induces electronic spin and charge transitions in the spin-polarized droplet. We show how these spin-related transitions can be observed in transport and coherent resonant tunnelling through the QSET.