Market efficiency is a key concept in finance and economics that refers to the degree to which prices in financial markets fully reflect all available information. In an efficient market, prices should accurately and quickly adjust to new information, making it difficult for investors to consistently outperform the market through stock picking or market timing strategies. There are three forms of market efficiency - weak, semi-strong, and strong - which differ based on the types of information that are already reflected in market prices. Research in market efficiency examines the efficiency of different financial markets and the implications for investor behavior, market regulation, and financial decision-making.