An efficient market is a market where prices reflect all available information. In an efficient market, it is difficult for investors to consistently achieve Excess Gain that are higher than average because prices adjust quickly to new information. There are three forms of efficient markets: weak, semi-strong and strong form. Weak-form efficiency suggests that current stock prices fully reflect past prices, semi-strong efficiency suggest that prices fully reflect all publicly available information, and strong form efficiency suggests that prices fully reflect all information, including insider information.