Simple iterations. Spectral radius and convergence. Diagonal dominance case. M-matrix case. Preconditioners. Krylov subspaces and minimization of residual. Geometric and algebraic versions. Optimality of Krylov subspaces. Error estimates of Elman's type. Error estimates using polynomials and resolvents. Asymptotic convergence rate and logarithmic potential. Preconditioners and spectral equivalence. Superlinear convergence. Spectral clusters and equidistribution.