A consulting firm or simply consultancy is a professional service firm that provides expert advice for a fee. Consulting firms may have one employee or thousands; they may consult in a broad range of domains, for example, management, engineering, and so on.
Management consultants, in particular, typically work with company executives and provide them with generalists and industry-specific specialists, known as subject-matter experts, usually trained in management or in business schools. The deliverable of a management consultant is usually recommendations for achieving a company objective, leading to a company project.
Many consulting firms complement the recommendations with implementation support, either by the consultants or by technicians and other experts. This is called outsourcing.
Consulting services are part of the tertiary sector and account for several hundred billion dollars in annual revenues. Between 2010 and 2015, the 10 largest consulting firms alone made 170 billion dollars growth revenue and the average annual growth rate is around 4%.