Overall performance of a system is always limited by its lowest performing sub-system. Of course, this rule is transferable to energy-autonomous micro and nano systems for which all the elements must be energy efficient. In this chapter, we will focus on the first sub-system of a node of a wireless sensor network, the sensor and its proximity electronic. These electronics, situated closest to the sensor, transform if necessary, physical information into an electric signal (voltage, current, frequency...) and/or amplify it so that it is directly compatible with a digital data conversion. It should be noted that, in certain cases, the analog-to-digital conversion is an integral part of the electronic proximity (ΣΔ for example). Due to the low supply voltage and little energy available in the node of an autonomous sensor, the sensor and its electronic front-end must therefore be the least energy-consuming as possible. In this context, capacitive transduction sensors occupy a large part in the development of micro- and nano-sensors, both in scientific literature and in commercial success. After reviewing some generalities about the first electronic stage of an autonomous sensor node, most of this chapter will therefore be dedicated to capacitive sensors. The second part addresses resistive transduction, often considered incompatible with low power consumption constraints even if recent works allow this to be reconsidered. In conclusion, we will sum up this chapter by emphasizing the main transduction methods adapted to ultra-low power applications.