As the techniques presently used for routine measurements of nanoparticle size are largely unsatisfactory, especially for supported metal catalysts, new guidelines and procedures are described that allow us to obtain more accurate and reliable values of nanodimensions by chemisorption. Improved values of turnover frequencies will result, with benefit for studies on the mechanism of catalytic reactions. Examples taken from both old and recent studies show that nanoscience has been applied in catalysis for a long time. It is quite probable that an extensive use of nanotechnologies in catalyst R-D will bring unexpected industrial achievements in the future. Meanwhile, nanoscience principles can be profitably applied to the improvement of many current petrochemical catalysts.