Rapid development of computing power of personal computers, workstations, mainframes, super computers and integrated circuits has provided scientists and engineers with powerful tools in solving their scientific problems using computers, and is expected to continue to increase well in the future. The monograph mainly contains the following three parts: analysis of supremum of weighted pseudoinverses, study the stability of weighted pseudoinverses, weighted least squares problems and constrained weighted least squares problems, and stable methods for solving weighted least squares problems and constrained weighted least squares problems.