a grass cutting device suitable for automatic and unattended operation , comprises at least one drive wheel ; a power unit operatively coupled to said drive wheel to propel said device ; a cutter mechanism to cut grass over which said device passes and obstacle detection means capable of detecting an obstacle in the path of said device and being operably linked to said power unit to cause said device to retreat and turn away from said obstacle . the grass cutting device preferably further comprises mechanical long grass detection means arranged to detect long grass in the path of said device .