A consultant is a professional who provides expert advice in a particular area such as security (electronic or physical), management, education, accountancy, law, human resources, marketing (and public relations), finance, engineering, science or any of many other specialized fields. Internal consultant: someone who operates within an organization but is available to be consulted on areas of their specialization by other departments or individuals (acting as clients)

External consultant: someone who is employed externally to the client (either by a consulting firm or some other agency) whose expertise is provided on a temporary basis, usually for a fee. Consulting firms range in size from sole proprietorships consisting of a single consultant, small businesses consisting of a small number of consultants, to mid- to large consulting firms, which in some cases are multinational corporations. This type of consultant generally engages with multiple and changing clients, which are typically companies, non-profit organizations, or governments.

By hiring a consultant, clients have access to deeper levels of expertise than would be financially feasible for them to retain in-house on a long-term basis. Moreover, clients can control their expenditures on consulting services by only purchasing as much services from the outside consultant as desired.

Consultants provide their advice to their clients in a variety of forms. Reports and presentations are often used. However, in some specialized fields, the consultant may develop customized software or other products for the client. Depending on the nature of the consulting services and the wishes of the client, the advice from the consultant may be made public, by placing the report or presentation online, or the advice may be kept confidential, and only given to the senior executives of the organization paying for the consulting services