We present a new open-source Python package, krotov, implementing the quantum optimal
 control method of that name. It allows to determine time-dependent external fields 
for a wide range of quantum control problems, including state-to-state transfer, 
quantum gate implementation and optimization towards an arbitrary perfect entangler.
Krotov's method compares to other gradient-based optimization methods such as 
gradient-ascent and guarantees monotonic convergence for approximately 
time-continuous control fields. The user-friendly interface allows for combination 
with other Python packages, and thus high-level customization.