This chapter presents the principles of asynchronous iterative methods and shows how these methods can be applied to sub-structuring methods. Asynchronous iterative methods are not used much today, but new needs arise from new hardware architectures and asynchronous methods become a suitable solution for large scale computation. The major reason is that these methods are much less dependent on the interconnection latency and bandwidth. This chapter first introduces the asynchronous concept on simple iterative algorithms. Next the extension to asynchronous sub-structuring methods is presented. Numerical results, showing the robustness and efficiency of these asynchronous sub-structuring methods, are finally presented.