Mechanical properties and measurements of conjugated polymers have been introduced by Professor Lipomi in volume II. In this chapter, we mainly focus on the materials design principles for conjugated polymers that are mechanically and electronically suitable for stretchable electronics. Thin films of conjugated polymers that present excellent electronic properties are typically mechanically fragile, while polymers that offer excellent deformability and stretchability do not show excellence in electronic properties. The major efforts in search of conjugated polymers for stretchable electronics are to lift this trade-off between their electronic and mechanical properties. There are currently two successful engineering approaches, namely strain engineering and the adoption of nanocomposites (i.e. the nanoconfinement effect). In strain engineering, conjugated polymers are placed onto an elastic substrate to form complex thin film structures that can be reversibly deformable under strain. In the nanocomposites, conjugated polymers are dispersed into elastic matrix polymers. Through the nanoconfinement effect, the blended thin films can present substantially improved stretchability of polymer semiconductors without diminishing their electronic properties (i.e. charge transport mobility). Other than these two engineering approaches, a third approach from polymer chemists aims to minimize the trade-off and design intrinsically stretchable conjugated polymers that meet the needs of both electronic and mechanical requirements. Both side chain and backbone engineering have been demonstrated to be effective in the design of intrinsic stretchable conjugated polymers.