Dynamical degrees of rational transformations

Consider a rational transformation $f$ of a projective variety $M$, and iterate $f$ to get a sequence of rational transformations $f^n$ The dynamical degree of $f$ is a positive real number that describes the exponential growth rate of the sequence $\deg(f^n)$, where the $\deg(f^n)$ is the degree of the formulae defining $f^n$. I shall describe properties of these numbers, and list a few open problems.