FastCam is an instrument jointly developed by the Spanish Instituto de Astrofísica de Canarias and the Universidad Politécnica de Cartagena designed to obtain high spatial resolution images in the optical wavelength range from ground-based telescopes. The instrument consists of a very low noise and very fast readout speed EMCCD camera capable of reaching the diffraction limit of medium-sized telescopes from 500 to 850 nm. FastCam incorporates a FPGAs-based device to save and evaluate those images minimally disturbed by atmospheric turbulence in real time. The undisturbed images represent a small fraction of the observations. Therefore, a special software package has been developed to extract, from cubes of tens of thousands of images, those with better quality than a given level. This is done in parallel with the data acquisition at the telescope. After the first tests in the laboratory, FastCam has been successfully tested in three telescopes: the 1.52-meter TCS (Teide Observatory), the 2.5-meter NOT, and the 4.2-meter WHT (Roque de los Muchachos Observatory). The theoretical diffraction limit of each telescope has been reached in the I band (850 nm) -0.15, 0.08 and 0.05 arcsec, respectively-, and similar resolutions have been also obtained in the V and R bands. Future work will include the development of a new instrument for the 10.4-meter GTC telescope on La Palma.