The paper is a survey of the latest results on quasi-stationary phenomena in nonlinearly perturbed stochastic systems. New methods of asymptotic analysis for nonlinearly perturbed stochastic processes based on new type of asymptotic expansions for perturbed renewal equation and recurrence algorithms for the constructing of of asymptotic expansions for Markov type processes with absorption are presented. Asymptotic expansions are given in mixed ergodic (for processes) and large deviation theorems (for absorption times) for nonlinearly perturbed regenerative processes, semi-Markov processes, and Markov chains. Applications to analysis of quasi-stationary phenomenon in nonlinearly perturbed queuing systems, population dynamics and epidemic models, and for risk processes are presented.