This project will design novel modelling frameworks by using time delay to represent chemical events with multi-step reactions. The developed modelling approaches will be employed to develop stochastic models for telomere length regulation in ageing research. The central aim of this project is to investigate the origin and function of heterogeneity in telomere length as well as to explore molecular mechanisms that regulate telomere reduction and replication. This project will also propose effective and efficient numerical methods for simulating chemical reaction systems with time delay. A rigorous theory will be established for analyzing the convergence property of numerical methods for simulating chemical systems with time delay.