The state-of-the-art for CMOS Application Specific Integrated Circuits (ASIC) and their applications will be described stressing the implications on circuit design and introducing the concept of Systems-on-Chip (SoC), as well as some simulation and design (including layout) tools for ASICs. Different mixed signal front-end circuits for Integrated Circuits (IC) and ASICS will be described and the intended applications. These descriptions will be used to classify circuits in terms of functionality, power consumption, size and figures-of-merit such as linearity, noise immunity and so on, of capital importance for ICs and ASCIs design. Finally, techniques for power consumption reduction and performance enhancement will be presented through several circuit examples.