This thesis considers three empirical areas of seasoned equity offering (SEO): (1) SEO stock price reaction, (2) the determinants of SEO underpricing and (3) the post-issuing performance of SEO. During the period 1999 to 2006 in Thailand, we identified 126 companies issuing via common stocks, which illustrated four different issuing methods. We found in the first context that there is a negative reaction of stock prices to SEO announcements. The issuing methods can reduce the offering dilution caused by these announcements. We also noticed that there is no relationship between short- and long-term stock price reactions. In the context of the determinants of SEO underpricing, the evidence reported contrasting aspects to those noted in the existing literature, particularly in the area of asymmetric information. Nevertheless, some factors that we examined provided different discussions regarding trading. Finally, we found underperformance of SEO firms during the post-issue period. This confirms the results when a different methodology is employed.