This thesis examines the performance of IPO firms in the Swedish markets during their first three years of trading to investigate what effects different factors have on the short- and long-term performance. The level of underpricing on Swedish IPOs are also investigated to detect any abnormalities from previous studies on larger markets in the US and Europe. A total of 175 IPO firms included in the sample in the period between 2000 and 2015. The method used to calculate the aftermarket performance is the buy-and-hold abnormal returns method in an event-time portfolio approach. These returns are used as a dependent variable in a multivariable linear regression analysis. The main findings from this study are that IPO firms in the Swedish markets underperform the OMX market index one month and three years after the offering. Conversely, IPO firms in the Swedish markets overperform after 18 months compared to the OMX market index. The results show that Swedish IPOs are on average underpriced and that underpricing has a significant effect on short-term performance. Firm age, offer size and the technology industry also have a significant effect on the first month’s performance of IPO firms; however, there is little support for these factors to affect the longterm performance.