Clemens Hoischen has been awarded the H.E.S.S. Prize for his path-finding leadership of the H.E.S.S. transients program. Clemens designed and implemented a new, more flexible transient alert system, which is based on the VO standard and replaced the old GCN alerter in September 2016. The VO alert system receives external alerts, processes and filters them, and initiates prompt follow-ups of the alerts. Clemens connected H.E.S.S. to multiple sources of alarms (IceCube and Antares for neutrinos, Ligo/Virgo for gravitational waves), opening up completely new possibilities for the transients physics programs, which resulted in 2017 in the follow-up observations of the gravitational wave neutron star-neutron star merger and the follow up of the neutrino event IC-EHE-170922. In both cases, H.E.S.S. was the first VHE instrument to be on target. Thanks to Clemens, today H.E.S.S. has a modern and flexible transient system, unparalleled in ground-based Cherenkov astronomy and state-of-the-art compared to other wavelengths. In addition, Clemens also developed and established data unblinding and analysis strategies for transient sources, and initiated the systematic analysis and planned publication of all GRB data that has been taken since 2012. Since 2017 he leads the H.E.S.S. transients envelope task, which is responsible for the coordination of all transient-related activities.