A state or group possessing biological and chemical agents must overcome wide-ranging technical challenges to actually use them in anger.
1. Storage and transport
Some chemical and biological agents degrade if exposed to heat, light, moisture or oxygen, so must be stored and transported in inert conditions - and without dangerous leaks.
So-called binary chemical weapons store two relatively safe precursor chemicals separately, mixing them just before use.
Stabilising additives can be added to some agents - but this adds to the cost and procurement difficulties.
There is debate over whether unaccounted for or suspected undeclared Iraqi agent stocks would have degraded by now. Inspectors are also unsure what progress Iraq made in stabilising the VX nerve agent it produced.
2. Air defences
Bombs dropped from planes, ballistic missiles and cruise missiles have been designed to carry chemical and biological agents. But all these would need to overcome the target's air defences.
Planes are vulnerable to detection, as are unmanned drones. Either could be fitted with crop sprayers, but would then have to fly quite low.
Ballistic missiles, such as Iraqi Scuds, cannot be guided once launched, increasing the likelihood of them being detected and shot down.
Some analysts say they also travel too fast and get too hot to deliver agents very effectively.
Cruise missiles hug the terrain as they travel and fly low to avoid radar detection, so are less likely to be intercepted.
Ground-fired munitions avoid the possibility of interception, but the short range puts friendly troops at greater risk form the agent once it is released.
Most biological and chemical agents enter the body through inhalation, apart from nerve agents which can penetrate the skin in liquid form and mustard, which also attacks the skin.
But most agents are liquid at room temperature, so delivery methods need to aerosolise the agent into tiny droplets or heat it to form vapour which could then be breathed in.
Explosives can disperse and heat the agent, but a substantial proportion of it may be burnt up in the explosion, or, in the case of biological agents, destroyed by the heat.
Weapons using automatic spraying systems have also been designed, but are more complex - so are more expensive and less readily available.
If the agent is released too high in the atmosphere, it will drift upwards, so some delivery systems uses timers or sensors to trigger the release just before the weapon lands.
To read more about different types of agents, use the Biochemical Weapons section of the menu on the top right of this page.
Military chemical and biological attacks would be likely to attempt to disperse an agent across a large target like a city, rather than to pinpoint a single building as modern precision bombing aims to do.
But, despite this often wide margin of error, accuracy can still be a problem.
If shells or missiles designed to carry solid explosives are adapted to deliver liquid agents, the motion of the agent during flight can knock the weapon off course.
Also, some biological agents are degraded by the high acceleration of a shell or missile.