In this thesis we study the properties of strongly-coupled large-N conformal field theories (CFT’s) using AdS/CFT correspondence. Chapter 1 serves as an introduction. In Chapter 2 we study the shear viscosity of strongly-coupled large-N conformal field theories. We find that it is affected by \(R^2\) corrections to the AdS action and present an example of 4D theory in which the the conjectured universal lower bound on viscosity-to-entropy ratio \(\eta/s > 1/4 \pi\) is violated by 1/N corrections. This fact proves that there is no universal lower bound of \(1/4 \pi\) on viscosity-to-entropy ratio and may be relevant for the studies of QCD quark-gluon plasma for which this ratio is experimentally found to be close to \(1/4 \pi\). In Chapter 3 we study the formation of the electron star in 4D AdS space. We show that in a gravity theory with charged fermions a layer of charged fermion fluid may form at a finite distance from the charged black hole. We show that these “electron stars” are candidate gravity duals for strongly interacting fermion systems at finite density and finite temperature. Entropy density for such systems scales as \(s \sim T^{2/z}\) at low temperatures as expected from IR criticality of electron stars solutions. / Physics