This thesis describes three-terminal transport measurements through single molecules. The interest in this field stems from the dream that single molecules will form the building blocks for future nanoscale electronic devices. The advantages are their small size -nanometers-, and their synthetic tailorability which allows the molecules to be designed with built-in functionality. Many efforts in experimental research are nowadays devoted to understand electrical transport though single molecules. Several techniques are used for this purpose: scanning tunneling microscopy (STM), mechanical controllable break junctions (MCBJ), atomic force microscopy (AFM), and shadow evaporation. The method used throughout our work to fabricate nanogaps is named electromigration. We show that after trapping a molecule in the nanogap, a molecule at low temperatures behaves as a quantum dot (QD), i.e., a system where electrons can be placed in discrete energy states. The control over the number of electrons that can sit on the molecule is achieved by the voltage on a third electrode, the gate, which is capacitively coupled to the QD. Transport measurements, which consist of taking current-voltage characteristics as a function of this gate voltage, reveal important spectroscopic information about the molecule.