Electronic Journal of Differential Equations, Vol. 2008(2008), No. 145, pp. 1-13. Title: Stability and approximations of eigenvalues and eigenfunctions for the Neumann Laplacian, part I Authors: Rodrigo Banuelos (Purdue Univ. West Lafayette, IN, USA) Michael M. H. Pang (Univ. of Missouri, Columbia, MO, USA) Abstract: We investigate stability and approximation properties of the lowest nonzero eigenvalue and corresponding eigenfunction of the Neumann Laplacian on domains satisfying a heat kernel bound condition. The results and proofs in this paper will be used and extended in a sequel paper to obtain stability results for domains in $\mathbb{R}^2$ with a snowflake type boundary. Submitted February 4, 2008. Published October 24, 2008. Math Subject Classifications: 35P05, 35P15. Key Words: Stability; approximations; Neumann eigenvalues and eigenfunctions.