The adult slug measures 100 to 130 millimetres. It has a slender body, narrowing towards the tail. Variable in colour, it may be pale grey, ash coloured, brownish or sometimes yellowish white, streaked or spotted with black. It also has a black-spotted, oblong shield that is around one-third of its total length. The sole of the foot is ash coloured or yellow, and it has long, slender tentacles. The mucus is colourless and iridescent.
Almost always found near human habitation, slugs typically live in lawns, gardens, greenhouses, cellars, tunnels and other damp places. In natural areas, they are restricted to deciduous and mixed forests and can be seen hiding under pieces of timber, in tree hollows and bark, and under rocks. The great grey slug is common throughout Europe, except the far north, thriving in damp places with above-zero temperatures.
These slugs feed at night on dead plants, fungi, fruit and vegetables, with a preference for windfall and rotting fruit.