This thesis describes the thermoelectric properties of mesoscopic structures in modulation doped GaAs/ Al0.33Ga0.67As heterostructures. In particular, we study the thermopower of two-dimensional mesoscopic systems (2DMSs) and antidot lattices defined in them. Quantum oscillations in the thermopower (TP) of a quantum point contact allow accurate determination of the local electron temperature. The low temperature (T < 0.3 K) TP of square (5 ?m x 5 ?m) mesoscopic devices shows a highly enhanced value, almost two orders of magnitude larger than estimates from the free electron model. The TP is sensitive to the Fermi energy, showing strong fluctuations which are correlated with anomalous structures in non-equilibrium transport. The non-monotonic temperature dependence of TP shows similarities with Kondo lattice systems, thus suggesting the existence of localized spins in the 2DMS. At low magnetic fields , the Nernst coefficient in these 2DMSs shows unexpected oscillations, which are commensurate with those of the RudermanKittel- Kasuya-Yosida exchange. To study spin effects in a periodic potential, we investigate transport and thermopower in antidot lattices (ADLs). These ADLs are completely controlled by electrostatic gating. Commensurability features in magnetoresistance show that the antidot potential and Fermi energy of the system can be tuned independently. The magneto-thermopower shows an even richer structure, displaying AharonovBohm type interference effects. A detailed study of the zero-field TP allows us to study the interplay between the antidot potential and the Fermi energy. The results of this thesis show that thermoelectric studies provide a powerful way to probe spin-correlations and energy dependent scattering processes in mesoscopic structures in GaAs/ AlGaAs systems.