They are the most vital substance of the plant.
Essential oils are very potent, and many are too strong to use directly on the skin.
If you use them for massage, add them to massage oil or cream, and experiment with the amount to use. Even in a bath, too much of a particular oil could be irritating to your skin. Begin by using just a few drops. If you like the effect, you can gradually add a bit more if you wish. Some oils can counteract homeopathic remedies, so be sure to check with a homeopath if you are using both treatments simultaneously.