openEMS is an electromagnetic field solver using the FDTD method. The tool can be used to design and understand the electromagnetic behavior of antennas, filters, PCBs and more. It has an Octave (or Matlab) and Python interface to setup and analyze the simulation model. This allows for a great flexibility and the possibility to integrate or interface to other software. The talk will give a short introduction into the FDTD method and for which cases it is suitable, which features it offers, how to get started using the tool and which interfaces to other software already exist.