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In this thesis, we investigate all warped AdS$_4$ and AdS$_3$ backgrounds with the most general allowed fluxes that preserve more than 16 supersymmetries in 10- and 11-dimensional supergravities. Assuming either that the internal manifold is compact without boundary or that the isometry algebra of the background decomposes into that of AdS and that of the transverse space, we find that there are no AdS$_4$ backgrounds in IIB supergravity. Similarly, we find a unique such background with 24 supersymmetries in IIA supergravity, locally isometric to $AdS_4\times \mathbb{CP}^3$. In 11-dimensional supergravity all more than half BPS AdS backgrounds are shown to be locally isometric to the maximally supersymmetric $AdS_4\times S^7$ solution. Furthermore, we prove a non-existence theorem for AdS$_3$ solutions preserving more than 16 supersymmetries. Finally, we demonstrate that warped Minkowski space backgrounds of the form $\mathbb{R}^{n-1,1}\times_w M^{D-n}$ ($n\geq 3,D=10,11$) in 11-dimensional and type II supergravities preserving strictly more than 16 supersymmetries and with fields, which may not be smooth everywhere, are locally isometric to the Minkowski vacuum $\mathbb{R}^{D-1,1}$. In particular, all such flux compactification vacua of these theories have the same local geometry as the maximally supersymmetric vacuum $\mathbb{R}^{n-1,1}\times T^{d-n}$.