  We investigate the relation of the semigroup probability density of an
infinite activity L\'{e}vy process to the corresponding L\'{e}vy density. For
subordinators, we provide three methods to compute the former from the latter.
The first method is based on approximating compound Poisson distributions, the
second method uses convolution integrals of the upper tail integral of the
L\'{e}vy measure and the third method uses the analytic continuation of the
L\'{e}vy density to a complex cone and contour integration. As a by-product, we
investigate the smoothness of the semigroup density in time. Several concrete
examples illustrate the three methods and our results.
