We first survey componentwise and normwise perturbation bounds for the standard least squares (LS) and minimum norm problems. Then some recent estimates of the optimal backward error for an alleged solution to an LS problem are presented. These results are particularly interesting when the algorithm used is not backward stable.

The QR factorization and the singular value decomposition (SVD), developed in the 1960s and early 1970s, remain the basic tools for solving both the LS and the total least squares (TLS) problems. Current algorithms based on Householder or Gram-Schmidt QR factorizations are reviewed. The use of the SVD to determine the numerical rank of a matrix, as well as for computing a sequence of regularized solutions, is then discussed. The solution of the TLS problem in terms of the SVD of the compound matrix (b A) is described.

Some recent algorithmic developments are motivated by the need for the efficient implementation of the QR factorization on modern computer architectures. This includes blocked algorithms as well as newer recursive implementations. Other developments come from needs in different application areas. For example, in signal processing rank-revealing orthogonal decompositions need to be frequently updated. We review several classes of such decompositions, which can be more efficiently updated than the SVD.

Two algorithms for the orthogonal bidiagonalization of an arbitrary matrix were given by Golub and Kahan in 1965, one using Householder transformations and the other a Lanczos process.