The stock market is undoubtedly a successful tool for providing companies with the necessary capital. However, developments in the stock markets are shaped by the behaviors of the investors who provide the capital. Emotions, expectations, intentions, and abilities of individuals determine market trends. History shows how people's feelings can influence prices. Euphoria and panic are often followed by booms and crashes. This makes it clear that there is not only a goal-oriented approach among individuals, but also uncontrolled and sometimes uncontrollable emotions present in the investment process. This book lists and analyzes central psychological factors that hinder rational investment decisions. In particular, it examines through the evaluation of various studies whether experts are more strongly influenced by these so-called biases than laypersons, which has led to some surprising results.