Abstract. This paper gives explicit rational functions for interpolating and approximating functions on the intervals [-1,1], [0, oo], and [-oo, oo]. The rational functions are linear in the functions to be approximated, and they have preassigned poles. The error of approximation of these rationals is nearly as small as the error of best rational approximation with numerator and denominator polynomials of the same degrees. Regions of analyticity are described, which make it possible to tell a priori the accuracy which we can expect from this type of rational approximation.1. Introduction and Summary. In this paper we attempt to give a constructive, affirmative answer to each of the following questions.1. Given a function / and an interval /, is it possible to tell a priori whether or not one can accurately approximate / via a low-degree rational function?2. Can such a rational function be easily constructed explicitly, so that one encounters no poles on the interval of approximation?3. Can one use the Thiele algorithm to construct or evaluate this rational function? 4. Can one tell a priori when we can expect the Thiele algorithm, the e-algorithm, or the Padé method to produce an accurate low-degree rational approximation?5. Does the error of this rational function compare favorably with the error of the best possible rational approximation of the same degree?Although we cannot give an affirmative answer to the above questions in all cases, we shall describe classes of analytic functions which house nearly all of the cases encountered by the author in applications, and for which the answer to each of the above questions is "Yes".We shall develop a class of rational approximations for interpolation over [-1,1], [0, oo], and [-oo, oo]. These rational approximations share many of the features of SINC methods summarized in [23]. The interpolation points of these rationals are the same as the SINC interpolation points, and the classes of functions which low-degree rationals approximate accurately are the same as the classes which the SINC functions approximate accurately.Indeed, the error bounds for, e.g., approximation on [-1,1] of functions analytic on the unit disc are the same as the SINC bounds, i.e., rationals have the same optimality properties as SINC methods. In using rationals instead of SINC functions, we lose many of the simple relations that SINC functions satisfy, such as