This chapter (partially based on: (i) Levin MS (2010) Towards communication network development (structural system issues, combinatorial problems). In: Proceedings of IEEE Region 8 International Conference SIBIRCON-2010, vol. 1. pp 204–208 with permission from IEEE (ii) Levin MS (2012) Towards design of hierarchy (research survey). Electronic preprint, p 36, Accessed 8 December 2012 http://arxiv.org/abs/1212.1735 [math.OC] (iii) Levin MS (2013) Improvement/extension of modular systems as combinatorial reengineering (survey). Electronic preprint, pp 24, Accessed 17 April 2013 http://arxiv.org/abs/1304.4965 [cs.AI]) addresses frameworks for development (or improvement, extension) of composite (modular) systems (as combinatorial reengineering). The system improvement/extension strategies are examined as selection/combination of the improvement action(s)/operation(s) and as modification of system structure. The following approaches and applications are described: (i) system improvement by system components; (ii) reoptimization in combinatorial optimization (improvement of an existing solution for optimization problem at previous time stage while taking into account cost of the improvement process and proximity of the obtained new solution to a solution for the optimization at new time stage); (iii) modification of system structures (e.g., modification of tree via condensing of vertices, hotlink assignment problems, transformation of tree into Steiner tree, transformation of morphological structure, transformation of layered structure); (iv) real-world examples for communication network improvement/extension.