The purpose of this book is to provide a clear understanting of the ACO, by giving a proper and complete systematization of the subject. The simulation developed in .NET using C# language will be a support of a deeper analysis of the factors of the ACO based algorithm, its implementations in routing algorithm, mostly for switched-based telecommunication network,for analysis of network performance will be explained.Simulation results using ACO mode willbe compared with non-ACO mode using graphs. Conclusions of recent studies will be given and resume the current employments of this great algorithm inspired by the Nature. All networks tend to become more and more complicated. They can be wired, with lots of routers, or wireless, with lots of mobile nodes.The problem remains the same: in order to get the best from the network, there is a need to find the shortest path as well as less congested path. The more complicated the network is, the more difficult it is to manage the routes and indicate which one is the best. The Nature gives us a solution to find the shortest path. The real ants, in their necessity to find food and brings it back to the nest,lay down a chemical pheromone.