During the past decade the understanding of photo-induced ultrafast dynamics in molecular systems has improved at an unforeseen speed and a wealth of detailed insight into the fundamental processes has been obtained.

This review summarizes our present knowledge on ultrafast dynamics in isolated molecules and molecular clusters evolving after excitation with femtosecond pulses as studied by pump–probe analysis in real time. Experimental tools and methods as well as theoretical models are described which have been developed to glean information on primary, ultrafast processes in photophysics, photochemistry and photobiology. The relevant processes are explained by way of example—from wave packet dynamics in systems with a few atoms all the way to internal conversion via conical intersections in bio-chromophores. A systematic overview on characteristic systems follows, starting with diatomic and including larger organic molecules as well as various types of molecular clusters, such as micro-solvated chromophore molecules. For conciseness the focus is on molecular systems which remain unperturbed by the laser pulses—apart from the excitation and detection processes as such. Thus, only some aspects of controlling and manipulating molecular reactions by shaped and/or very intense laser pulses are discussed briefly for particularly instructive examples, illustrating the perspectives of this prospering field.

The material presented in this review comprises some prototypical examples from earlier pioneering work but emphasizes studies from recent years and covers the most important and latest developments until January 2006.