Final Activity Report Summary - PC-NANOSCOPY (Application of photochromic compounds to improve resolution in fluorescence microscopy up to molecular scale) We developed new photochromic diarylethene compounds that could be dissolved and function in aqueous environment, with the final aim of utilising them as fluorescence bistable compounds to achieve resolutions of a few nanometres in Reversible saturable optical fluorescence transitions (RESOLFT) techniques, applied on biological samples. We also designed, prepared and characterised new fluorescent switchable probes based on the photochromism of rhodamine amides' derivatives. This kind of compounds was successfully applied in nanoscopies, based on the switch-on and localisation of single molecules with optical sectioning capabilities. Probes with different characteristics were designed, in order to achieve various goals such as photoactivation with continuous wave lasers, multicolour applications, e.g. for colocalisation experiments, and environment probing of areas of the sample based on the mobility of the probe. All of the goals were achieved with resolutions far beyond the diffraction limit of 10 to 20 nm. The proof of the proposals' utility was demonstrated, in particular in biological samples. The principle to apply regular fluorophores in nanoscopies based on the localisation of single molecules was presented. Cell imaging with biomolecular resolution was also presented, with commercial fluorophores and visible fluorescent proteins.