This review discusses electron transport through double quantum dots (DQDs), focusing on their potential for quantum computing. The study covers experimental results on DQDs fabricated at Delft University of Technology and NTT Basic Research Laboratories. The review highlights the importance of DQDs in realizing solid-state quantum bits (qubits) due to their ability to mimic artificial atoms and molecules. The paper begins with an introduction to quantum dots, their structure, and the role of Coulomb blockade in electron transport. It then discusses the stability diagram, which visualizes the equilibrium charge states of two serially coupled dots. The stability diagram is a key tool for analyzing DQD transport properties. Resonant tunneling experiments show that the width of resonances is determined by the lifetime of discrete energy states, independent of electron temperature. The study also explores level spectroscopy in a magnetic field, demonstrating the ability to resolve avoided crossings in the spectrum of a quantum dot. Microwave spectroscopy is used to probe the transition from ionic to covalent bonding in DQDs, revealing the character of inter-dot coupling and quantifying bonding strength. The review also discusses the non-linear transport regime, where multiple discrete energy levels can enter the bias window, leading to resonances within conductance triangles. The role of discrete levels in determining transport properties is emphasized, with experiments showing that the spacing of current steps reflects the energy spacing of levels. Resonant tunneling through DQDs is illustrated with a schematic potential landscape, showing how electrons can tunnel through discrete energy levels. The study demonstrates that the relaxation rate to the ground state is not necessarily higher than the tunnel rate through the dot, and that transport through excited states can play a significant role. The review concludes with a discussion of magnetic field spectroscopy, where energy evolution of states near the Fermi energy is measured. The results show crossings and anti-crossings between Coulomb peaks, revealing intra-dot level repulsion in quantum dot systems. The experiments are performed in the weak coupling limit, where mixing between quantum states is negligible. The study provides valuable insights into the behavior of DQDs and their potential for quantum computing applications.This review discusses electron transport through double quantum dots (DQDs), focusing on their potential for quantum computing. The study covers experimental results on DQDs fabricated at Delft University of Technology and NTT Basic Research Laboratories. The review highlights the importance of DQDs in realizing solid-state quantum bits (qubits) due to their ability to mimic artificial atoms and molecules. The paper begins with an introduction to quantum dots, their structure, and the role of Coulomb blockade in electron transport. It then discusses the stability diagram, which visualizes the equilibrium charge states of two serially coupled dots. The stability diagram is a key tool for analyzing DQD transport properties. Resonant tunneling experiments show that the width of resonances is determined by the lifetime of discrete energy states, independent of electron temperature. The study also explores level spectroscopy in a magnetic field, demonstrating the ability to resolve avoided crossings in the spectrum of a quantum dot. Microwave spectroscopy is used to probe the transition from ionic to covalent bonding in DQDs, revealing the character of inter-dot coupling and quantifying bonding strength. The review also discusses the non-linear transport regime, where multiple discrete energy levels can enter the bias window, leading to resonances within conductance triangles. The role of discrete levels in determining transport properties is emphasized, with experiments showing that the spacing of current steps reflects the energy spacing of levels. Resonant tunneling through DQDs is illustrated with a schematic potential landscape, showing how electrons can tunnel through discrete energy levels. The study demonstrates that the relaxation rate to the ground state is not necessarily higher than the tunnel rate through the dot, and that transport through excited states can play a significant role. The review concludes with a discussion of magnetic field spectroscopy, where energy evolution of states near the Fermi energy is measured. The results show crossings and anti-crossings between Coulomb peaks, revealing intra-dot level repulsion in quantum dot systems. The experiments are performed in the weak coupling limit, where mixing between quantum states is negligible. The study provides valuable insights into the behavior of DQDs and their potential for quantum computing applications.