MARXS (Multi-Architecture-Raytrace-Xraymission-Simulator) is a toolsuite to simulate X-ray observatories. It is primarily aimed at astronomical X-ray satellites and sounding rocket payloads, but can be used to ray-trace experiments in the laboratory as well. MARXS performs polarization Monte-Carlo ray-trace simulations from a source (astronomical or lab) through a collection of optical elements such as mirrors, baffles, and gratings to a detector. MARXS modular structure is designed to serve two main use cases: Build-your-own instrument: Instrument designers can construct any X-ray experiement from a set of building blocks such as mirrors, diffraction gratings and CCD detectors. Simulate science observations: Given an instrument configuration, simulate the detector output for any set of X-ray sources in the lab or on the sky. MARXS is written in Python.