The Casimir effect is a prediction of Quantum Field Theory at the interface with open problems of modern physics such as the puzzle of vacuum energy. The ideal expression written by Hendrik Casimir for perfect mirrors is not suffícient for comparison between theory and experiments. The effects of the optical properties of mirrors, thermal fluctuations and dissipation, plane-sphere geometry, surface irregularities, have to be taken Ínto account. I will discuss the remaining discrepancy between theoretical predictions and the most precise experiments performed at IUPUI with metallic mirrors in vacuum . I will also evoke ongoing experiments performed at UFRJ with spherical díelectric particles ín electrolytes .