The consulting industry has developed into one of the most mature sectors in the professional services industry, generating between $100 billion to $300 billion in revenues, with the precise estimate depending on the definitions used. At the heart of the industry stand six main domains – Strategy Consulting, Management Consulting, Operations Consulting, Financial Advisory, HR Consulting and IT Consulting – that when combined, span services and offerings in over 200 industry and functional areas. The majority of consultants work at large and mid-sized consulting firms, yet, in terms of the number of consultancy enterprises, these firms typically represent no more than 15% of the total, with the majority of enterprises active as sole proprietorships (i.e. freelance consultants).