This chapter discusses designing System-on-Chips (SOCs) with maximum flexibility, where most of the system is controlled by software, enabling easy customization for specific applications. It explores how customized processors and hardware intellectual properties (IPs) enhance performance while maintaining adaptability, making software-defined SOCs highly efficient. The chapter covers the approach to designing application-specific hardware, highlighting the integration of Application-Specific Instruction Processors (ASIPs) and programmable accelerators to support evolving software requirements. It also addresses key challenges such as hardware-software partitioning, co-verification, and system integration, ensuring reliability and security. Additionally, the chapter delves into Software-Defined Vehicles (SDVs) and other dynamic applications, emphasizing the role of EDA tools, FPGA prototyping, and ASIC design methodologies in enabling next-generation SOC architectures.