Abstract. We present eigenvalue bounds for perturbations of Hermitian matrices, and express the change in eigenvalues in terms of a projection of the perturbation onto a particular eigenspace, rather than in terms of the full perturbation. The perturbations we consider are Hermitian of rank one, and Hermitian or non-Hermitian with norm smaller than the spectral gap of a specific eigenvalue. Applications include principal component analysis under a spiked covariance model, and pseudo arclength continuation methods for the solution of nonlinear systems.Key words. eigenvalues, Hermitian matrix, eigenvalue gap, perturbation bounds, non-Hermitian perturbations, principal components, numerical continuation.AMS subject classification. 65F15, 65H10, 65H17, 65H20, 15A18, 15A421. Introduction. We present perturbation bounds for eigenvalues of Hermitian matrices that were motivated by two applications: principal component analysis under a spiked covariance model [25], and pseudo arclength continuation methods for the solution of systems of nonlinear equations [7].Although these applications are very different, they share a common requirement: The change in the eigenvalues of interest should be determined not by the global norm of the full perturbation, which can be quite large, but rather by the norm of a projection of the perturbation on a particular eigenspace. In contrast, most existing eigenvalue bounds are expressed either in terms of the full perturbation or else in terms of a residual, and therefore do not provide sufficient information for our applications.The paper is organized as follows. We start with the most specific class of perturbations, Hermitian rank-one updates, and then generalize the perturbations first to Hermitian and then to non-Hermitian matrices. In §2 we present bounds for Hermitian rank one updates, and explain why such bounds can be useful in pseudo-arclength continuation methods. In §3 we consider Hermitian perturbations whose norm is smaller than the spectral gap of a specific eigenvalue, and describe their use in principal component analysis. In §4 we extend the bounds to non-Hermitian perturbations.