Nanotechnology is a key element in modern technological progress offering solutions to some of the most pressing issues for society, such as in the fields of health, communication, and energy consumption. The solutions that nanotechnology provides comprise amplifiers, actuators, or sensors and emitters, etc. as building blocks operating at the nanoscale. Semiconductor nanowires are perfectly suited for the use in such applications, as they not only provide functional electronic and optical properties but also nanoscale dimensions. Their high aspect ratio furthermore makes them inherently suited to bridge scales from the nanoscale to the macroscopic scales humans interact with. Key requirements for such nanodevices-besides the high functionality-are also reliability and stability. A thorough understanding of the operation and the limitations of nanowire-based devices is therefore urgently needed. Here, the operation, stability, and degradation of optoelectronic and all-optical devices based on semiconductor nanowires are investigated. Ultra-high spatial resolution hard X-ray detection is demonstrated using a GaAs nanowire with an axial p-n junction. Advanced X-ray analytical techniques are applied to study the detector operation by monitoring internal electrical fields in-operando. Inspecting the stability, a hot electron beam-induced selective oxidization of the n-type nanowire segment is observed. Further, the operation and stability of all-optical ZnO and CdS nanowire devices are investigated. The spontaneous and stimulated emission from the nanowires is detected as a function of electron irradiation using an electron microscope. As electron beams are frequently used for nanoscale sample inspection and fabrication, an understanding is sought for electron beam-induced changes of the optical properties.