Rare B decays are an excellent laboratory for the search for physics beyond the Standard Model. In the last few years several new measurements from LHC and B-Factories experiments have been released with impressive experimental precision. In this talk, the most recent measurements of Rare B-Decays from the CMS collaboration will be reviewed. After a brief theoretical introduction, some details on the experimental strategy and on the analyses technique will be reported. Particular emphasis will be dedicated to the comparison of the experimental results with the predictions of the Standard Model as well of new physics models.