New rating issues and rating revision have always had a direct impact on the market price of financial instruments issued by the rated entity and the impact is normally higher when the information is unexpected.During the current financial crisis, the effect of any rating change was exacerbated by the higher level of uncertainty in the market and regulators are currently evaluating the best solution that can be implemented in order to avoid the negative effect of bank panic or financial market speculation.The chapter presents a detailed analysis of the effect of new ratings and rating changes on issuers and issues, discussing also the main motivations proposed in the literature in order to explain the phenomenon. Starting from the analysis of the current regulatory framework, the analysis proposed evaluates on a 10-year time horizon the changes registered in the upgrades and downgrades released by different types of raters and discusses the impact of the new regulation changes currently under evaluation in the main worldwide countries.