Spurred on by extensive research in recent years, organic semiconductors are now used in an array of areas, such as organic light emitting diodes (OLEDs), photovoltaics, and other optoelectronics. In all of these novel applications, the photoexcitations in organic semiconductors play a vital role. Exploring the early stages of photoexcitations that follow photon absorption, "Ultrafast Dynamics and Laser Action of Organic Semiconductors" presents the latest research investigations on photoexcitation ultrafast dynamics and laser action in pi-conjugated polymer films, solutions, and microcavities. In the first few chapters, the book examines the interplay of charge (polarons) and neutral (excitons) photoexcitations in pi-conjugated polymers, oligomers, and molecular crystals in the time domain of 100 fs - 2 ns. Summarizing the state of the art in lasing, the final chapters introduce the phenomenon of laser action in organics and cover the latest optoelectronic applications that use lasing based on a variety of cavities, such as distributed feedback-type cavity. With contributions from a host of renowned international experts, this book explores the underlying processes in both existing and potential organic optoelectronic applications. It provides a broad overview of the scientific debate in the field of photophysics in organic semiconductors.