The paper analyses explicit and implicit algorithms for incomplete factorization. The structure of preconditioning matrices and properties of their entries are considered as well as the conditions for the correctness of methods and for the convergence of iterative processes. The convergence rate estimates for iterations are given, and the results of numerical experiments are discussed.The incomplete factorization iterative methods have a thirty-year long history formally started in 1958 by publishing in [1] the Buleev method (later considered and developed in [2][3][4][5][6] and also in other papers) of solving a linear system of five-point grid equationsapproximating the boundary value problem for the second order elliptic equation. In (0.1), the boundary conditions are assumed to be taken into account at the values of coefficients a ij9 b ij9 c ij9 d ij9 e itj and of right-hand sides f itj . All coefficients are assumed to be non-negative, and the coefficient matrix is assumed to possess the properties of diagonal predominance, i.e. e itj ^ a itj + b itj + c itj + d ij9 and at least in one case the inequality is strict. It means that, since the coefficient matrix A is indecomposable, this matrix is monotonic (to be more precise, A belongs to the class of M-matrices; see [4]).Under the natural row-by-row ordering of nodes, the matrix of the system can be presented in the formwhere D = {Dj} is a block diagonal matrix, each block being a tridiagonal matrix with non-zero entries -a i>j9 e itj9 -c itj9 while L-{Lj} and U = {£/,·} are, correspondingly, lower and upper triangular matrices with non-zero entries b itj and d itj . If we exploit the fact that the grid nodes are ordered by diagonals, A is also a block tridiagonal matrix but now in equality (0.2), D is a diagonal matrix, while L and t/, in the general case, have in the rows two non-zero entries, each. The incomplete factorization methods were given rise to and developed on the basis of the approximation of A by a matrix K which can be presented in the form of a product of easily invertible matrices. The iterative process can be reduced to the formwhere u n = {w? tj ·}, /={//.,/}, and τ η are parameters determined from the Chebyshev acceleration, the conjugate gradient method and others. The diversity of choice of preconditioning matrices has resulted in a large number of publications containing different algorithms of incomplete factorization. These publications can be divided into two groups. The first group deals with the