Mars lacks a global dynamo magnetic field to shield it from the solar wind and solar storms, so may be especially sensitive to changing space weather compared to Earth. Inputs from the Sun and solar wind have been measured continuously at Mars for 20 years, and intermittently for more than 50 years. Observations of the influence of the variable space weather at Mars include compression and reconfiguration of the magnetosphere in response to solar storms, increased likelihood of aurora and increased auroral electron energies, increased particle precipitation and ionospheric densities during flare and energetic particle events, and increased ion escape during coronal mass ejection events. Continuing measurements at Mars provide a useful vantage point for studying space weather propagation into the heliosphere, and are providing insight into the evolution of the Martian atmosphere and the role that planetary magnetic fields play in helping planets to retain habitable conditions near their surface.