This chapter introduces the basic concepts of mechanical design and its general relation with the properties derived from tensile testing. It begins with a description of the basic objective of product design. Next, a simple tie bar is used to illustrate the application of mechanical property data to material selection and design and to highlight the general implications for mechanical testing. Material subjected to the basic stress conditions is considered to establish design approaches and mechanical test methods, first in static loading and then in dynamic loading and aggressive environments. The chapter then briefly describes design criteria for some basic property combinations such as strength, weight, and costs as well as stiffness in tension. Additionally, it describes the processes involved in mechanical testing for stress at failure and elastic modulus. Finally, the chapter examines the correlation between hardness and strength.