Ant colony optimization (ACO) has become one of the popular metaheuristics used for tackling optimisation problems. Its popularity grows steadily, and the area of applications constantly widens. Recent research shows that ACO is competitive and applicable to various real-world optimisation problems. ACO has been initially developed to handle only the discrete optimisation problems. In this work, we present a way to extend ACO, so that it can also be applied to both continuous and mixed-variable optimisation problems. We initially focus only on the continuous case. Following an extensive analysis of the performance of ACO extended to continuous domains, we present how it may be further adapted to handle both continuous and discrete variables simultaneously. We thus introduce the first native mixed-variable version of an ACO algorithm. The book is recommended for all those interested in novel approaches to heuristic optimisation and ways to practically tackle difficult optimisation problems. The ideas described are tested on real-world example problems, which allows better understanding and demonstrates the advantages of the approach.