In this thesis we have solved the two-dimensional problem of locomotion of modular robots on square lattices in the presence of obstacles of arbitrary forms, for the class of modular robots that follow the model of movement known as the sliding-cube model. The algorithm we propose is deterministic and distributed. All robot modules apply the same set of local rules. That is, they make their own decisions based on local information from their immediate surroundings. The result is a scalable algorithm that solves the problem efficiently. Our results close a problem that was open for more than ten years.