This presentation concerns the metrology and applications of low frequency noise in the field of very high frequency electronics. Noise metrology has its own constraints, and its own mathematical relationships to translate the fine mechanisms that define the electronic detection thresholds. The presentation will take place in two parts: one dedicated to the metrology of LF noise (LFN) measurements and the other related to their applications. The optimization and definition of an experimental device for the LFN depend on its specifications, which itself depends on the targeted limits to be measured or the ease of use for the targeted buyer. The definition of the measurement setup (and their associated equations), the compromises of settings (gain, bandwidth, RF-DC decoupling and current and impedance levels) are given by the presentation of different setups. A comparison of the advantages, drawbacks and limitations is given between home-made and commercial setups. User-friendly ease of use and detection noise floor are primarily defined by the trade-off between the ability to drive the DC signal generator or to bias the DUT from batteries. Many performances that define the quality of the LFN measurement arise from these RF-DC decoupling considerations. The second part of the presentation focuses on the applications of these LF noise measurements; it distinguishes the design of RF circuits from the study of noise sources and their evolution during the application of stresses (DC, Thermal, RF) for reliability studies. The design of high purity oscillators (MIC and MMIC) is a major concern for telecom, radar or radiometer applications. This presentation provides some intuitive design guidelines for BiCMOS (VCO MMIC) and GaN (fixed oscillator MIC and VCO) technologies. Phase noise is defined as the distance between the electrical power of the carrier (signal) and the noise at a given frequency of the carrier. SiGe technology is the best choice when it comes to LF noise and its NL conversion of phase noise, but GaN technologies offer an increased carrier signal by one to two decades and can also be considered despite higher LF spectra. The last part of the presentation concerns the use of LFN measurement to locate defects within an active device (HBT, HEMT). It also presents the use of noise spectroscopy to study the dependence of a defect versus the application of a stress (evolution of the spectra and specific monitoring of the electrical signature of the noise). The LFN represents a powerful tool to develop reliability studies, since it represents a non-invasive measurement (however, we need to DC bias the device and likely to perform measurements at different temperatures for the Arrhenius plots!). Chosen case of study are given as illustrative purpose on both SiGe HBT technologies and GaN HEMT technologies.