Engineers apply the theories and principles of science and mathematics to the economical solution of practical technical problems. Often their work is the link between a scientific discovery and its application. In addition to design and development, many engineers work in testing, production or maintenance. They supervise production in factories, determine the causes of breakdowns, and test manufactured products to maintain quality. They ... estimate the time and cost to complete projects. Some work in management or sales where an engineering background enables them to discuss technical aspects of a product and assist in planning its installation or use.