  In this article, we consider the geodesic flow on a compact rank $1$
Riemannian manifold $M$ without focal points, whose universal cover is denoted
by $X$. On the ideal boundary $X(\infty)$ of $X$, we show the existence and
uniqueness of the Busemann density, which is realized via the
Patterson-Sullivan measure. Based on the the Patterson-Sullivan measure, we
show that the geodesic flow on $M$ has a unique invariant measure of maximal
entropy. We also obtain the asymptotic growth rate of the volume of geodesic
spheres in $X$ and the growth rate of the number of closed geodesics on $M$.
These results generalize the work of Margulis and Knieper in the case of
negative and nonpositive curvature respectively.
