I'll review how the stability of simulations in numerical relativity is related to having a well-posed continuum problem, and why well-posedness is not a property of the Einstein equations as such, but of the way in which they are formulated as an initial-boundary value (time evolution) problem. After reviewing the related concepts of well-posedness, strong hyperbolicity and symmetric hyperbolicity, I discuss applications to the Einstein equations, and in particular recent work. I conclude with an outlook on what strategies seem most promising to achieve well-posedness.