The electrical penetration graph (EPG) is the electrically recorded presentation of mouthpart penetration by piercing arthropods, so far mainly herbivorous homopterans. In these insects, the EPG reflects phenomena inside the stylet canals and in the plant tissue near the stylet tips; phenomena which alter the electrical properties between the electrodes, 1 on the insect and 1 in the plant. Most of these phenomena, which cannot be studied by any other technique, comprise the basic insect–plant interactions during stylet penetration and form the heart of homopteran–plant relationships. The measuring principle of the original AC system is based on amplitude modulation (AM) in which a low alternating voltage (from a weak alternating current, AC) with a high frequency is supplied to the plant. At the amplifier input, its amplitude fluctuates in accordance with (is modulated by) conductivity changes in the insect–plant combination. Signal processing then converts these amplitude changes into a signal matching the original conductivity changes, discarding the AC frequency. The AM principle makes the AC system a conductivity measuring device (i.e., the AC EPG is a record of the fluctuating voltage over time caused by changes in the electrical resistance of the insect–plant combination). The DC system, developed later, supplies the plant with a low constant voltage (from a weak direct current, DC), which at the amplifier input also fluctuates in accordance with (is modulated by) the insect–plant conductivity changes but, additionally, the DC amplifier measures weak electrical potentials actively generated by the insect–plant combination; these are called the electromotive forces (emf). For example, well-known electromotive forces are the transmembrane potentials of plant cells. Thus, the DC EPG is a record of the fluctuating voltage over time caused by changes in the electrical resistance as well the electromotive forces in the insect–plant combination. We can distinguish, therefore, that the insect–plant interactions evoke 2 measurable characteristics causing 2 signal components: the R component (resistance or conductivity component) and the emf component. Both components contain relevant biological information. The AM principle prevents emf components from being recorded because they do not affect (modulate) the amplitude of the AC frequency used. Therefore, an AM device is very suitable to record only the R component. All actual AC systems studied in this chapter do not optimally match the requirements of the AM principle for signal processing, but their actual specifications do not make them suitable for recording any emf component. The DC system is derived from amplifiers used for electrophysiological measurements. Because of an extremely high input resistance (10¹²–10¹⁵ Ω) electrophysiological amplifiers avoid R components. However, in the normal DC system for EPGs, the input resistance is lowered (10⁹ Ω) so that both components are recorded. In addition to the electrical origin of the signal, its frequency spectrum is important. For both the R component and the emf component in the EPG, the frequency spectrum ranges from 0 to 40 Hz. The frequency responses for AC and DC EPG systems are discussed with respect to this spectrum. Also, the frequency response requirements of recording devices (chart and tape recorders and/or computer acquisition systems) are evaluated. It is concluded that the current AC systems and most of the recording devices used with these AC systems (before computerization in recent years), chart recorders in particular, have a frequency response which is too low to record the full frequency spectrum. The biological importance of recording both electrical origin components (R and emf) and a full frequency range for the insects concerned seems rather clear from studies done so far with DC systems. However, for specific studies, the use of an optimized AM based AC system, recording the R component without a superimposed emf signal, or the use of a very high-input resistance DC system, recording the emf component without a superimposed R signal, can be attractive scientifically. Also, filtering specific frequency ranges can be of great help to clarify certain details or waveforms. Therefore, the possible combination of an optimized AM based AC system and a DC system in a single device is discussed as an attractive option. Such a combination could simultaneously measure the R component at 1 output (AC) and R+emf components at a 2nd output (DC), which could be changed by a switch into a pure emf signal, if desired. Such a device can provide a useful combination for future research and development of EPG recording, and may turn the present AC–DC controversy into collaboration on biological issues.