Steffen studied biology in Heidelberg and did his doctoral thesis with the group of Thomas Cremer. A post-doc with Renato Paro in Heidelberg was followed by a post-doc with Andrew S. Belmont at the University of Illinois at Urbana-Champaign. Following that, he started as junior group leader at the Faculty of Biology at the Ludwig-Maximilans-Universität München (LMU), where he completed his Habilitation. At all these stations he worked on nuclear architecture, which required high resolution confocal microscopy and live-cell microscopy. He then moved to the Walter-Brendel-Zentrum of the LMU where he became head of light microscopy. Here he got intensely involved in multi-photon fluorescence excitation and Third Harmonic Generation (THG) microscopy. When the new Biomedical Center of the LMU started operations in fall 2015, he became head of the Core Facility Bioimaging.