A consulting firm or simply consultancy is a professional service firm that provides expertise and specialised labour for a fee, through the use of consultants. Consulting firms may have one employee or thousands; they may consult in a broad range of domains, for example, management, engineering, and so on. Consultant can work on a project-by-project basis or for months or years as a contractor. Consultants can also have as many clients as they have the capacity for.