Nowadays, around the world, in the competition for gaining market share, companies are trying to increase their share of sales against their competitor's. This research investigates the effects of market share changes on the market’s valuation of future earnings and growth opportunities. This study was conducted for the time span of 2001-2006 using a sample of 168 among companies that listed in Tehran Stock Exchange (TSE). This paper develops two measures of market share and assesses the valuation implications of a firm's change in market share and the momentum and direction of that change, for current and future performance. The results show that the link between current performance and future performance is significantly enhanced in the presence of market share gains. Further, this relation holds for up to three years into the future. In addition, this study employs two models to assess the current valuation implications of a firm's change in market share and momentum for current performance using unexpected returns and price models. The results find evidence consistent with current earnings being more value relevant when accompanied by growth in market share. But, this research indicates that there is no significant relation between market share changes and future growth opportunities.