  Porosity and dimension are two useful, but different, concepts that quantify
the size of fractal sets and measures. An active area of research concerns
understanding the relationship between these two concepts. In this article we
will survey the various notions of porosity of sets and measures that have been
proposed, and how they relate to dimension. Along the way, we will introduce
the idea of local entropy averages, which arose in a different context, and was
then applied to obtain a bound for the dimension of mean porous measures.
