Pipe bombs vary in sophistication but even the most basic can cause serious damage when thrown through windows, left outside a door or pushed through a letter-box.
The most simple variety is made from a length of metal tubing, sometimes only a foot long. The tubing is usually copper, lead or the mild steel used for scaffolding. The pipe is sealed with solder at one end and filled with gunpowder or fireworks.
The devices can be detonated by using batteries, percussion caps, or simply a piece of cloth lit by a match.
Slightly more advanced versions have been discovered which are usually larger and sometimes have screw-on caps attached to the ends.
Examples of a third and more sophisticated type of pipe bomb were discovered by RUC officers in Ballymena, Co Antrim, at the end of June.
These are known as "keyhole" pipe bombs. The devices, with keys attached, were designed to hang from keyholes on the door of targeted houses.
The idea is that the device will rest at head height to cause maximum damage to the person who opens the door.
"There's no special skill required," said an RUC source.