The submarine operates in three dimensions and whilst it has operating constraints in common with the surface ship it has others peculiar to the added dimension. This paper, which was presented at a meeting of the Institute in London on 18 December 1974 with Captain R. Maybourn in the Chair, discusses these constraints as they affect submarine safety and attempts to identify those which are caused by modern commercial activities. The views expressed are those of the author and should not be taken as a statement of Ministry of Defence policy.