In this study, we propose constructive ways to determine input-to-state stability (ISS) as well as incremental ISS (ISS) of nonpolynomial dynamical systems. The developed procedures are based on sums-of-square decomposition. This tool is only applicable to polynomial systems. Thus, a rational recast of the nonpolynomial system description is used. This recast generally leads to an increased system order and additional constraints. These constraints must be respected in the resulting formulations. The proposed approach gives a unique and constructive procedure to determine the ISS and the ISS property, which is normally nontrivial and needs a good understanding of the system's dynamics. The proposed approaches are illustrated on several examples. K E Y W O R D S nonlinear feedback control, input-to-state stability, Lyapunov methods, sum-of-squares programming Abbreviations: ISS, input-to-state stability; ISS, incremental input-to-state stability; SOS, sums-of-squares. [Correction added on 3 December 2020, after first online publication: Rick Voßwinkel was designated as corresponding author.] This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.