Despite the success of the standard model (SM) of particle physics, a few unsolved problems call for new physics beyond the SM. In this thesis, we focus on one theoretical problem of the SM---Planck-electroweak (EW) hierarchy problem, as well as two experimental facts---non-zero neutrino mass and observed baryon asymmetry, which can not be explained by the SM.

One plausible solution to the Planck-EW hierarchy problem is Randall-Sundrum (RS) models with a warped extra dimension, or their AdS/CFT dual composite Higgs (CH) theories. Moreover, it is well-known that type I seesaw mechanism can naturally generate tiny but non-zero SM neutrino mass. The same seesaw mechanism can also explain the baryon asymmetry of the universe via leptogenesis. Therefore, in order to address these three problems in a single model, we study a warped/composite seesaw model, a natural embedding of the high scale type I seesaw mechanism in RS/CH framework. In contrast to the usual high scale type I seesaw mechanism in four dimensions (4D), 5D warped seesaw model becomes a TeV scale ``inverse'' seesaw like model after Kaluza-Klein decomposition into 4D theories. In order to study leptogenesis in warped/composite seesaw, we first develop a simplified version of this model, as part of a general framework called hybrid seesaw. We then demonstrate that hybrid seesaw can achieve successful leptogenesis and feature an interesting interplay of high scale generation of the asymmetry and TeV scale washouts, which has a larger viable parameter space and richer phenomenology than usual type I seesaw models. To make this mechanism realistic in the full warped seesaw model, we also study the phase transition from the high temperature black hole phase to low temperature phase with two branes in 5D theories. According to AdS/CFT duality, this phase transition is dual to the deconfined and confined phase transition in strongly coupled nearly conformal 4D theories. It was previously believed that this phase transition rate is too slow at the critical temperature, resulting in a large amount of supercooling and low scale inflation. All primordial abundance in this case would be significantly diluted. We analyze a new mechanism to achieve fast phase transition around the critical temperature and thus the asymmetry generated from high scale leptogenesis can survive until today.