This lecture delves into the dominance of the 'Big Four' audit firms - KPMG, Deloitte, PricewaterhouseCoopers (PwC), and Ernst Young (EY) - in the global auditing, accounting, and corporate consulting industry. It explores the history of these firms, their repeated failures in auditing, and their involvement in profit-shifting practices. The lecture also discusses the substantial profit margins of for-profit exchanges, the conflicts of interest between auditing and consulting, and the impact of high-frequency trading (HFT) on financial markets, shedding light on the secretive world of HFT players and strategies. Additionally, it covers the role of Securities Information Processors (SIPs) and order types in trading environments.