Josef Kallrath is a specialist in modeling and solving mixed integer linear and nonlinear optimization problems. He offers consultancy services since 1998 in various industries (among them process industry, paper and metal industries, and energy). His service includes the model building process, implementation of the mathematical model in a modeling language such as GAMS, solving the problem using standard solvers, and, if standard solvers are not sufficient, the construction of tailored solution algorithms. His current research interests are polylithic modeling approaches to solve large-scale or difficult optimization problems, for instance, by decomposition techniques such as column enumeration and column generation, or by hybrid methods. The customer base is not limited to his home country Germany, but also includes reference customers from the US, England or Japan. Josef Kallrath also offers GAMS modeling and optimization courses, as well as in-house course adjusted to client's wishes.