Quantum dots, nanometer-scale semiconductor structures that can be charged with electrons, can exhibit excited-state transitions that mimic those of atoms (see the Perspective by McEuen, p. 1729). Kouwenhoven et al. (p. 1788) show that in circular, defect-free quantum dots, quantum numbers and transitions corresponding to Hund's rule in atoms can be identified. Schedelbeck et al. (p. 1792) studied the formation of artificial molecules from such artificial atoms. The overlap between the wave functions from two quantum dots led to the formation of bonding and antibonding levels whose coupling strength depended on the distance between the dots. Stewart et al. (p. 1784) studied irregular quantum dots to investigate electronic transitions in systems lacking symmetry. Strong correlations in excitation spectra were observed for quantum dots with different numbers of electrons, despite the difference in charging energies, and one electron, rather than two electrons with paired spins, were sufficient to fill a level.