A consultant is usually an expert or a professional in a specific field and has a wide knowledge of the subject matter.[2] A consultant usually works for a consultancy firm or is self-employed, and engages with multiple and changing clients. Thus, clients have access to deeper levels of expertise than would be feasible for them to retain in-house, and may purchase only as much service from the outside consultant as desired. A consultant is usually an expert or a professional in a specific field and has a wide knowledge of the subject matter.[2] A consultant usually works for a consultancy firm or is self-employed, and engages with multiple and changing clients.