the stock market was once a large room with traders shouting ”Buy” and ”Sell” at live monitors, this picture has now changed. In modern times, there are fewer traders and a lot more computers. The traders who control these computers can be sitting anywhere and these computers work much faster and smarter than humans. The programs that run these computers are based on mathematical models called algorithms. Instead of traders, it is these algorithms determine when a stock is bought or sold. Some of these algorithms are simpler than others, for example, the computer can be programmed so that when the price of Security A reaches a certain value Y, it is computed to buy/sell X amount of Security A. Others are more complex, incorporating thousands of separate models and codes and are able to execute various strategies within a fraction of a second, these are known as High-Frequency Trading Computers.