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The paper is a review on modelling and experimental results regarding microsphere resonators. The applications of the whispering-gallery-modes (WGMs) resonances in conventional glass microspheres to perform chemical spectroscopy, biomedical sensing, low threshold lasing are recalled. The behaviour of silica microsphere coated via silica hafnia 70SiO2-30HfO2 and activated with 0.3% mol Er3+ for NIR operation is described. The feasibility of amplifying systems, operating in Mid-IR wavelength and based on Er3+-doped Ga5Ge20Sb10S65 chalcogenide microspheres coupled to tapered a fiber is discussed. The investigation has been performed via a dedicated 3D numerical model, based on the coupled mode theory and solving the rate equations. The code includes the transitions among the erbium energy levels, the amplified spontaneous emission and the most important secondary transitions pertaining to the ion-ion interactions. The model follows the particle swarm optimization (PSO) approach, allowing both the amplifying system optimization and to perform the microsphere glass spectroscopy.