Many accountants work for small firms or as individual practitioners, but the sector is dominated by four massive firms, who compete for the biggest contracts and operate globally. These firms are Deloitte, PwC, EY and KPMG, collectively they are named the Big 4. Working for a Big 4 firm means you'll be working with the biggest clients, in an enviroment which offers a variety of projects, helping you develop a rounded skillset. 

The Big 4 firms offer services across the full range of accountancy tasks, including taxation services, consulting and risk assessment. Each has hundreds of thousands of employees spread across the globe and offer numerous opportunities for advancement and a structured enviroment in which to build a career. The environment in a major accounting firm isn’t for everyone, working for a smaller firm will suit many people, especially those looking to specialise in a sector, or who want to build deeper relationships with a smaller set of clients.

Ultimately, every firm big or small has their own culture, learning about their strengths and weaknesses is the best route to picking the right one for your career.