In the modern era of computing, microprocessors continue being at the heart of many computer systems. The different requirements by varieties of applications from edge devices, IoT, mobile, personal computers, and high-performance systems introduce major challenges on processor microarchitecture. These challenges have been further heightened when advance VLSI process technologies ceased from providing frequency scale to fuel processor performance growth. In modern systems, additional factors such as scale, power, and cost have become crucial design factors. As a result, all these challenges have intensified the importance of processor microarchitecture in conjunction with the physical constraints and application requirements. In this chapter, different processor microarchitectures are comprehensively covered. Through the discussion, the objective is to study design implementation details while examining hardware efficiency and performance. The chapter starts with a single-cycle processor and overview processor data-path and control unit. Next, the chapter introduces pipelined processors and presents the design complexity and performance aspects. Finally, multiple-issue processors such as superscalar, VLIW, and out-of-order processors are presented. The limitations and challenges of such processors to exploit the available parallelism of instructions are presented. The chapter is concluded by summarizing the future trends and challenges for processor microarchitectures.