In an efficient market, prices adjust rapidly to new information, rendering it impossible for investors to consistently achieve returns that exceed average market returns on a risk-adjusted basis. There are three forms of market efficiency: weak, semi-strong, and strong, each based on the types of information considered. Weak form efficiency asserts that all past trading information is reflected in stock prices. Semi-strong form asserts that all publicly available information is reflected in stock prices. Strong form efficiency posits that all information, public and private, is fully reflected in stock prices. The efficient market hypothesis (EMH) is a key theory in finance that supports the idea of market efficiency.