Earthquake data are today mostly digital which gives the analyst many options for viewing the data in different ways (filtering etc) and perform advanced signal processing tasks. The data itself often requires signal processing in order to do simple tasks like filtering when picking phases. In the analog world it was common to have high gain SP records and a low gain LP record (see Chapter 3), which can be considered recording the signal with 2 fixed filters. However, even a simple operation like filtering is not without problems since filtering can change the arrival time of a particular phase. One of the most used possibilities with digital data is being able to correct the data for the effect of the instrument and thereby being able to view the signals (time or frequency domain) as ground displacement, velocity or acceleration within the bandwidth where they have a signal level above the background noise (see Chapter 3). Many books have been written on general signal processing e.g. Oppenheim et al. (1998). In geophysics there is e.g. Kanasewich (1973). For digital seismology the most relevant is Scherbaum (2001) and Stein and Wysession (2003). Some examples are also found in NMSOP and Havskov and Alguacil (2006). In this chapter we will deal with the most common methods of signal processing except spectral analysis which is dealt with in Chapter 6.