Functional materials based on trivalent lanthanide ions are of interest due to their photoluminescent properties. Especially, Nd3+, Eu3+ and Yb3+ ions, in addition to the extremely important utilitarian aspect such as laser materials (Nd3+ and Yb3+) or red phosphors (Eu3+), as conditioned by a favorable combination of many physicochemical and optical properties, the second – no less interesting cognitive aspect is determining the correlation between luminescent properties and the structure of the material. Due to the nature of selected transitions, they are used as spectroscopic probes enabling the determination of the number of non-equivalent crystallographic positions occupied by activators in the network and the degree of ordering of the internal structure, which makes them suitable for optical sensing systems. Even the smallest changes structural in the environment of the activator caused by modification of the chemical composition or the transformation of material during controlled heat treatment can be spectroscopically monitored. These three rare earth ions are highly sensitive to external perturbations such as temperature, pressure, and chemical species, which make them interesting candidates for in-situ imaging and sensing, environmental monitoring, and biomedical applications. The lecture will present examples of applications.