Whilst it is perfectly safe to apply many of doTERRA's essential oils neat to the skin, it is always advisable to dilute them for a number of reasons.
Firstly, everyone is different and therefore your skin may react differently to mine. How your skin reacts over time may also change, depending upon what you have been doing, ingesting, or wearing for instance.
Secondly, many people think that if a couple of drops of Essential Oils are good for me, then surely 20 drops of the same oil will do me 10 times as much good - this is not true. Essential Oils are already concentrated during the distillation process. Remember when you were a kid and got nettle rash? You'd take a dock leaf that was growing nearby and rub that on the rash to stop the stinging. Well a drop of oil will be much more concentrated than using that leaf.
Thirdly, research has shown that diluting Essential Oils with a carrier oil, such as fractionated coconunt oil, actually improves the effectiveness of the Essential Oil.
"By diluting your essential oil with a carrier oil, you slow the rate of evaporation, enhance absorption into the skin, and protect sensitive or delicate skin. As you can see, dilution does not take away from the essential oil experience, but allows for more effective topical application." doTERRA ebook 'Topical Use of Essential Oils'.
Finally, dilution your Essential Oil with a carrier oil will make it last longer!
Some Essential Oils should never be applied neat to the skin;
These are known as hot oils and should always be diluted and tested on a small patch of skin first, always err on the side of caution with these oils.
Similarly, never use a Citrus Essential Oil (Lemon, Lime, Wild Orange, Bergamot, Tangerine etc) on any part of the skin that will be exposed to the sun during the next 12 hours, whether you have diluted the oil or not.
Citrus base Essential Oils will turn your skin highly photosensitive and your skin will burn, even with the UK sun. Just don't do it.