Experimental results on both serial and parallel double quantum dots are presented. The quantum dots are realized in the two-dimensional electron gas of an AlGaAs/GaAs heterostructure. By varying the inter-dot tunnel barriers, double quantum dots are tuned from the weak coupling limit to the strong coupling case. In serial double quantum dots, bonding and anti-bonding molecular states are probed by both direct tunneling and photon-assisted tunneling spectroscopy. In parallel double quantum dots, a coherent transport through the whole structure is verified through Aharonov-Bohm oscillations in the magnetotransport data. Furthermore, coherent molecular states are read out by a cotunneling current. Finally, a scheme for preparing and measuring a solid-state quantum bit is discussed.