After a brief introduction to low-power VLSI design, the design space of ASIP instruction set architectures (ISAs) is introduced with a special focus on important features for digital signal processing. Based on the degrees of freedom offered by this design space, a consistent ASIP design flow is proposed: this design flow starts with a given application and uses incremental optimization of the ASIP hardware, of ASIP coprocessors and of the ASIP software by using a top-down approach and by applying application-specific modifications on all levels of design hierarchy. A broad range of real-world signal processing applications serves as vehicle to illustrate each design decision and provides a hands-on approach to ASIP design. Finally, two complete case studies demonstrate the feasibility and the efficiency of the proposed methodology and quantitatively evaluate the benefits of ASIPs in an industrial context.