The chapter explores the nature of design and analysis in aircraft engineering, as well as the role of modeling, optimization, and digital technologies in aircraft design. The chapter shows how the decision-making process can be formalized and explains some scientific foundations of computer-aided design technology. In addition, the chapter presents some defining features of a system and introduces the concepts of systematic approach, system decomposition, and optimization. Finally, in this chapter, the reader will learn what differentiates parameters from characteristics and how both can be used in aircraft modeling and analysis at different levels.