This book sets the foundations for load-balancing algorithms of active objects which belong to a parallel application developed with the ProActive middleware. The main approach is called the ", Robin Hood + Nottingham Sheriff", load-balancing algorithm, which was validated in the context of computer grids through large-scale network simulations. Also, this book presents a model of Grids based on observation and measurement of the key characteristics for active- objects load-balancing: processing capacity and inter-resource communication latency.