Asymptotic expansions for stationary and conditional quasistationary distributions of nonlinearly perturbed birth-death-type semi-Markov models are presented. Applications to models of population growth, epidemic spread and population genetics are discussed.The initial perturbation conditions are formulated in the forms of Taylor asymptotic expansions for transition probabilities (of embedded Markov chains) and expectations of transition times, for perturbed semi-Markov processes.The algorithms are based on special time-space screening procedures for sequential phase space reduction and algorithms for re-calculation of asymptotic expansions, which constitute perturbation conditions for the semi-Markov processes with reduced phase spaces.The final asymptotic expansions for stationary distributions of nonlinearly perturbed semi-Markov processes are given in the form of Taylor asymptotic expansions.Models of perturbed Markov chains and semi-Markov processes, in particular, for the most difficult cases of perturbed processes with absorption and so-called singularly perturbed processes, attracted attention of researchers in the mid of the 20th century.An interest in these models has been stimulated by applications to control and queuing systems, information networks, epidemic models and models of mathematical genetics and population dynamics. As a rule, Markov-type processes with singular perturbations appear as natural tools for mathematical analysis of multi-component systems with weakly interacting components.We refer here to the latest books containing results on asymptotic expansions for perturbed Markov chains and semi-Markov processes, Stewart (