Essential oils are highly concentrated and can be harmful if not used carefully, so it is important to do a lot of research and learning before using them.  I also feel that it is important to err on the side of caution and to not use essential oils haphazardly.   I am not an expert, so you need to do your own research and consult your physician if you have a medical condition.  Essential oils are not going to replace proper medical care.

Some of the safety guidelines are:

• Never use undiluted on the skin
• be careful and watch for allergic reactions
• Some essential oils are phototoxic and can cause problems when applied and then you are exposed to sunlight
• Some essential oils should be avoided during pregnancy or when you have another health condition such as asthma or epilepsy
• Better to use less.  They are very concentrated and more is not better, it could be dangerous.
• Always keep them away from children.  They are very concentrated and can be dangerous when not used properly.
• Essential oils should not be taken internally without proper guidance and knowledge of safety.
• Keep away from fire as they are flammable.

There is a pretty comprehensive book on safety Essential Oil Safety: A Guide for Health Care Professionals-, 2e.

AromaWeb seems to have a pretty large selection of articles to read about and learn more.  The national Association for Holistic Aromatherapy has important safety guidelines.