  Black holes encountered in general relativity are characterized by spacetime
singularities hidden within an event horizon. These singularities provide a key
motivation to go beyond general relativity and look for regular black holes
where the spacetime curvature remains bounded everywhere. A prominent mechanism
achieving this replaces the singularity by a regular patch of de Sitter space.
The resulting regular geometries exhibit two horizons: the outer event horizon
is supplemented by an inner Cauchy horizon. The latter could render the
geometry unstable against perturbations through the so-called mass-inflation
effect, i.e., an exponential growth of the mass function. This chapter reviews
the mass-inflation effect for spherically symmetric black hole spacetimes
contrasting the dynamics of the mass function for Reissner-Nordst\"om and
regular black holes. We also cover recent developments related to the late-time
attractors induced by Hawking radiation which exorcise the exponential growth
of the spacetime curvature encountered in the standard mass-inflation scenario.
In order to make the exposition self-contained, we also briefly discuss basic
properties of regular black holes including their thermodynamics.
