This chapter addresses different aspects of soft sensors and their importance, especially for mineral processing plants It considers current state-of-the-art soft sensors and the future outlook of the industry, taking into account different classes of linear and nonlinear in-the-parameter soft sensor models, their design, and soft sensor shell systems for providing a robust operation in industrial environments. A geometrical approach to soft sensor modeling is given for nonlinear but linear in-the-parameter models. Inclusion of phenomenology in linear in-the-parameter models is highlighted. The chapter includes a review and examples from the mineral processing industry.