One of the most studied aspects of Mergers and Acquisitions (M&A) is their effect both at the time of the announcement and during the period after. Researchers mostly focus on the short-term effects of such agreements, without thoroughly examining the behaviour of shareholders in the long term, namely after conclusion of the agreements. The scope of the paper is to examine the effect of M&A agreements in the efficiency and overall performance of Greek companies involved in such agreements. This research was based on a sample of 20 announcements made by companies listed in the Athens Stock Exchange and published at the Athens Stock Exchange website during the 2004-2006 period. The evaluation of the Company Performance was carried out with the help of 4 key indicators, like Liquidity, Capital structure, Activity and Performance. Results are in line with previous studies showing that M&A agreements have a positive effect on the efficiency and overall performance of the companies involved, yet only in the short term; this effect is reversed in the long term, thus creating a negative effect.