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 11dimensional 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 × CP 3 . In 11dimensional supergravity all more than half BPS AdS backgrounds are shown to be locally isometric to the maximally supersymmetric AdS 4 × 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 R n−1,1 × w M D−n (n ≥ 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 R D−1,1 . In particular, all such flux compactification vacua of these theories have the same local geometry as the maximally supersymmetric vacuum R n−1,1 × T d−n .(at least those I have not already mentioned). I wish to thank by name Louise Anderson for being a true friend and colleague, Lucy Dove for eating my pizza despite her severe lactose intolerance, Magdalena Gebhart for being my younger somewhat high-strung sister, Colin Gorrie for being a true mensch and fellow "p.j.", Sebastian Hoof for jointly surviving Kennington Lane, Frances and Stephen Lipman for all their help and support, Tome Perrin for counteracting four years of sedentary lifestyle, and Maria Stillfried for being my younger somewhat calmer sister. I would like to thank Markus Dollmann for being the best friend anyone could wish for, he is a veritable (self-declared) bantersaurus rex I can always count on, and for that I am eternally grateful.I could not have completed this work without the unfaltering emotional support and love of Natasha Lipman. I thank her for being my muse, "voluntary" proofreader, despite her inexplicable dislike of Fraktur, and for always calming me down when it all seemed overwhelming.Finally, I would like to dedicate this thesis to my mother and late father, I owe them everything. My Maman encouraged me to follow my own interests from a young age, and I am grateful for knowing I can always count on her unwavering support and love. My Papschi is the person who first fostered my passion for physics and mathematics. I am sure he would have been delighted to read this thesis, and probably not understand a word of it.