Polymer ultrathin films have become a fascinating multidisciplinary area of research due to its endless applications in our day-to-day life. With reduction of film thickness to a dimension comparable to the chain length, the confinement effect makes polymer films ideal system to investigate novel properties of two dimensional physics. Advent of sophisticated experimental techniques facilitates proper characterization of these films though literature is inadequate focusing thermal properties and glass transition temperature (Tg) in ultrathin polymer films (<100nm), which remains a challenging area for basic studies. This book presents new analysis scheme based on x-ray reflectivity technique to determine temperature dependent thickness variations and describes the utility of the scheme through detail and successful investigations of phenomena like negative thermal expansion and distributed Tg in ultrathin films. This book also ventures another emerging field of polymer research to observe how ion implantation can modify the polymer films. The analyses and results discussed here should be especially helpful for professionals to shed more light in the basic research of polymer films.