Hi...this completely depends on the diffuser. Many 'nebulizing' diffusers have one fill a reservoir with water first, then add a few drops of essential oil to float on the top. This is great when you just want to utilize the 'aroma' aspect of essential oils.
Other diffusers use the essential oils alone, completely undiluted - examples are our wood-base nebulizing essential oil diffusers
. These output a controllable pure mist of the essential oil itself - so IF you're wanting to use the diffuser to output a 'therapeutic' concentration of oil, these are the way to go.