This study examines how diversification changes firms' key characteristics, which consequently alter firms' value. The reason why I focus on this topic is because of the mixed findings in literature about the valuation effect of diversification. This study offers deeper insights to the influence of diversification on important valuation factors that are already identified in finance literature. Specifically, it examines if diversification affects firms' information asymmetry problem, firms' cost of capital and cash flow, and firms' production efficiency. The study looks at both the financial industry and non-financial industry and the chapters are arranged in the following order. Firstly, empirical studies show that investors do not value BHCs' pursuit of non-interest income generating activities and yet these activities have demonstrated a dramatic pace of growth in the recent decades. An interesting question is what factors drive the discontent of the investors with the diversification endeavors of the BHCs in non-interest income activities. The first chapter examines the subject from the view point of information opaqueness, which is unique in the banking industry in terms of its intensity. We propose that increased diversification into non-interest income activities deepens information asymmetry, making BHCs more opaque and curtailing their value, as a result. Two important results are obtained in support of this proposition. First, analysts' forecasts are less accurate and more dispersed for the BHCs with greater diversity of non-interest income activities, indicating that information asymmetry problem is more severe for these BHCs. Second, stock market reactions to earning announcements by these BHCs signaling new information to the market are larger, indicating that more information is revealed to the market by each announcement. These findings indicate that increased diversity of non-interest income activities is associated with more severe information asymmetry between insiders and outsiders and, hence, a lower valuation by shareholder. Secondly, since Lang and Stulz (1994) and Berger and Ofek (1995), corporate literature has taken the position that industrial diversification is associated with a firm value discount. However, the validity and the sources of the diversification discount are still highly debated. In particular, extant studies limit themselves to cash flow effects, totally overlooking the cost of capital as a factor determining firm value. Inspired by Lamont and Polk (2001), the second chapter examines how industrial and international diversification change the conglomerates' cost of capital (equity and debt), and thereby the firm value. Our empirical results, based on a sample of Russell 3000 firms over the 1998-2004 period, show that industrial (international) diversification is associated with a lower (higher) firm cost of capital. These findings also hold for firms fully financed with equity. In addition, international diversification is found to be associated with a lower operating cash flow while industrial diversification doesn't alter it. These results indicate that industrial (international) diversification is associated with firm value enhancement (destruction). Given the fact that the majority of the firms involved in industrial diversification also diversify internationally, failing to separate these two dimensions of diversification may result in mistakenly attributing the diversification discount to industrial diversification. Thirdly, financial conglomerates have been increasingly diversifying their business into banking, securities, and insurance activities, especially after the Gramm-Leach-Bliley Act (GLBA, 1999). The third chapter examines whether bank holding company (BHC) diversification is associated with improvement in production efficiency. By applying the data envelopment analysis (DEA), the Malmquist Index of productivity, and total factor productivity change as a decomposed factor of the index, are calculated for a sample of BHCs over the period 1997-2007. The following results are obtained. First, technical efficiency is negatively associated with activity diversification and the effect is primarily driven by BHCs that did not diversify through Section 20 subsidiaries before GLBA. Second, the degree of change in diversification over time does not affect the total factor productivity change but is negatively associated with technical efficiency change over time. This latter effect is also primarily shown on BHCs that did not have Section 20 subsidiaries before GLBA. Therefore, it can be concluded that diversification is on average associated with lower production efficiency of BHCs, especially those BHCs without first-mover advantage obtained through Section 20 subsidiaries. These chapters explores the possible channels through which diversification could alter firms' valuation. They contribute to the literature by offering further knowledge about the effect of diversification.