This review discusses electron transport through double quantum dots coupled in series, focusing on the stability diagram, resonant tunneling, magnetic field spectroscopy, and microwave spectroscopy. The stability diagram, or honeycomb diagram, visualizes the equilibrium charge states of two serially coupled dots, showing hexagonal domains where the charge configuration is stable. Resonant tunneling experiments reveal that the resonance widths are determined by the intrinsic lifetime of discrete energy states, independent of electron temperature. Magnetic field spectroscopy allows for the study of intra-dot level repulsion, resolving avoided crossings in the spectrum of a quantum dot. Microwave spectroscopy, particularly photon-assisted tunneling, probes the transition from ionic to covalent bonding in a double dot artificial molecule, providing insights into the character of inter-dot coupling and the bonding strength. The review highlights the relevance of these studies for realizing solid-state quantum bits.This review discusses electron transport through double quantum dots coupled in series, focusing on the stability diagram, resonant tunneling, magnetic field spectroscopy, and microwave spectroscopy. The stability diagram, or honeycomb diagram, visualizes the equilibrium charge states of two serially coupled dots, showing hexagonal domains where the charge configuration is stable. Resonant tunneling experiments reveal that the resonance widths are determined by the intrinsic lifetime of discrete energy states, independent of electron temperature. Magnetic field spectroscopy allows for the study of intra-dot level repulsion, resolving avoided crossings in the spectrum of a quantum dot. Microwave spectroscopy, particularly photon-assisted tunneling, probes the transition from ionic to covalent bonding in a double dot artificial molecule, providing insights into the character of inter-dot coupling and the bonding strength. The review highlights the relevance of these studies for realizing solid-state quantum bits.