This chapter (Based on: Levin MS, Fimin AV (2012) Design of modular wireless sensor. Electronic preprint, p 7, Accessed 9 March 2012, http://arxiv.org/abs/1203.2031 [cs.SE]) addresses modular design and improvement of wireless modular sensor. The generalized design framework involves the following: (i) hierarchical system modeling, (ii) combinatorial synthesis (composition) (hierarchical morphological design), and (iii) aggregation of the obtained structured solutions into the resultant aggregated solution(s). Aggregation strategy is based on extension of “system kernel” (knapsack-like problem is used). A real-world numerical example for fire alarm wireless sensors illustrates the modeling and design problems.