The academic book "Transactions on Computational Science XXI," edited by Ajith Abraham, is the 21st edition of the journal of the same name and focuses on the topic of nature-inspired computing and its applications. This volume contains 15 complete research articles that address various aspects of nature-inspired computational methods. The topics covered include neurocomputing, evolutionary algorithms, swarm intelligence, artificial immune systems, membrane computing, computing with words, artificial life, and hybrid approaches. This collection provides a comprehensive insight into current research findings and developments in computational science and is aimed at professionals and students in the fields of engineering and IT.