Signal modification is an important function in many applications of vibration. The tasks of signal modification may include signal conditioning (e.g., amplification and analog and digital filtering), signal conversion (e.g., analog-to-digital conversion, digital-toanalog conversion, voltage-to-frequency conversion, and frequency-to-voltage conversion), modulation (e.g., amplitude modulation, frequency modulation, phase modulation, pulse-width modulation, pulse-frequency modulation, and pulse-code modulation), and demodulation (the reverse process of modulation). In addition, many other types of useful signal modification operations can be identified. For example, sample and hold circuits are used in digital data-acquisition systems. Devices such as analog and digital multiplexers and comparators are needed in many applications of data acquisition and processing. Phase shifting, curve shaping, offsetting, and linearization can also be classified as signal modification. This chapter describes signal conditioning and modification operations that are useful in vibration applications. Signal modification plays a crucial role in component interfacing. When two devices are interfaced, it is essential to guarantee that a signal leaving one device and entering the other will do so at proper signal levels (voltage, current, power), in the proper form (analog, digital), and without distortion (loading and impedance considerations). For transmission, a signal should be properly modified (by amplification, modulation, digitizing, etc.) so that the signal-to-noise ratio of the transmitted signal is sufficiently large at the receiver. The significance of signal modification is clear from these observations. The material covered in this chapter is intimately related to what has been discussed in the chapters on signal analysis and instrumentation (Chapter 4 and Chapter 8).