EY previously known as Ernst & Young is a multinational professional services network based in London, United Kingdom. Along with Deloitte, KPMG and PwC, it is one of the Big Four accounting firms. The EY network is composed of member firms of Ernst & Young Global Limited, a UK company limited by guarantee.

EY is one of the largest professional services networks in the world. It primarily provides assurance, tax, information technology services (including managed services in areas like Cybersecurity, Cloud, Digital Transformation and AI), consulting, and advisory services to its clients.

Ernst & Young Global Limited operates as a network of member firms which are structured as separate legal entities in a partnership, which has 395,442 employees in over 700 offices in more than 150 countries. The firm's current partnership was formed in 1989 by a merger of two accounting firms: Ernst & Whinney and Arthur Young & Co. It was named Ernst & Young until a rebranding campaign officially changed its name to EY in 2013, although this initialism was already used informally prior to its sanctioning adoption.

In 2023, EY was the seventh-largest privately owned organization in the United States, and EY has for 25 years been continuously ranked on Fortune magazine's list of the 100 Best Companies to Work For, longer than any other accounting firm. The firm has, however, repeatedly come under scrutiny for systemic issues in their training, hiring, and work culture.