Pentacene (Pn) is the highest mobility organic semiconductor known. It is a crystalline organic solid that can be deposited on a substrate by either vacuum sublimation, or by spin coating of a suitable soluble precursor followed by mild annealing. In this talk I will focus on studies of Pn thin film growth in vacuum, studies by in-situ Low Energy Electron Microcopy, Scanning Tunneling Microscopy, and Photo Electron Spectroscopy. We find that the molecular orientation in the growing film, as well as the nucleation density depend strongly on the choice and preparation of the substrate. For instance, grain sizes can vary from 0.1 micrometer to over 0.1 millimeter. Molecules can orient in-plane or out-of-plane, depending on the electronic structure of the substrate. On most substrates, grains show random azimuthal orientation, but recently we have succeeded in growing epitaxial pentacene films on a number of different substrates that hold promise for the fabrication of epitaxial organic thin film transistors with improved transport properties. In this talk I will review our results and recent progress.