The last 20 years, it was observed an unprecedented growth of R&D investment in many developed economies. This growth of R&D investment provided an ideal opportunity for researchers to consider the relation between R&D expenditures and subsequent earnings and more specifically the reflection of the market stock price in the value of R&D investments. The purpose of the research is to study if the association between R&D expenditures and the market stock price is due to a systematic mispricing or due to other omitted risk factors and if the immediate expensing of R&D is conservative or aggressive, relative to reporting under R&D capitalization. For that purpose, a sample of IT, Pharmaceutical and Telecommunication firms listed on Athens Stock Exchange (ASE) has been selected. These firms have a complete monthly stock return history to provide unbiased connection between earnings changes and excess returns. Finally,it is important to examine how the adoption of Financial Reporting Standards (IFRS) affects R&D expenses.