What smells make you feel happy? Perhaps the fine aromas of chamomile, lavender, or peppermint? Maybe you respond well to the exotic smells of sandalwood or sage. Scents have a greater impact on our mood than many of us realize. Aromatherapy is an ancient practice of using scents to improve mood and feeling, and is often used in alternative medicine. This practice is, however, subject to debate and caution.
Plant-derived essential oils are used in order to effect one's physical, and psychological well-being. Delivery techniques include topical application, massage, inhalation, and others. Aromatherapists, who are trained in the practice of aromatherapy, utilize blends of therapeutic essential oils to treat a specific disorder.
The Aromatherapy Debate
Some would argue that, like other alternative medicine techniques, aromatherapy is not a proven treatment method as it lacks rigorous clinical evidence. Aromatherapy has, on the other hand, been around since the dawn of civilization, mostly through the use of incense and topical-application methods such as perfume, ointments, etc.
Essential oils are not really oils, but are liquid aroma compounds that volatize at room temperature. Essential oils are typically prepared from natural sources such as plants leaves, roots, seeds, etc. In the previous century they were also prepared from a range of animal sources such as sperm whales, civet cats, beavers, musk deer, etc. Animal sources are, for the most part, no longer used, due to international treaties aiming to protect endangered animals.
Use Essential Oils With Caution
Essential oils should be used with caution above certain concentrations, since some are toxic through ingestion, inhalation, absorption through the skin, and other routes. If you are unfamiliar with these limits, then the best thing to do is to read the Material Safety Data Sheet (MSDS) for the specific compound. You can find MSDSs for a specific compound online through Google, Wikipedia, suppliers, etc. In addition, any given essential oil should not be used without being diluted in a fixed oil (such as olive oil, coconut oil, etc.) Without dilution, essential oils can cause skin rashes or other negative effects.
Standard Essential Oils in Aromatherapy
Essential oils such as lavender, jasmine, chamomile and peppermint are commonly used to treat stress, anxiety and depression. Some benefits that have been attributed to aromatherapy, such as clarity of mind, relaxation and depression, may actually arise from a placebo effect, rather than from any physiological effect.
Some common aromatherapy essential oils and their treatments:
- Lavender: Anti-stress, anti-anxiety and anti-depressant.  
- Chamomile: Soporific, insomnia, relaxant, inflammation, muscle spasms, menstrual disorders. 
- Sage: Boosts short-term memory performance. 
- Thyme: Mouthwash, antibiotic, fungicide.
- Jasmine: Anti-stress, anti-anxiety and anti-depressant.
- Sandalwood: Serenity, urogenital, and a skin antiseptic.
- Lemon: Uplifting and relieves stress.  
- Peppermint: Natural pesticide, soothe or treat symptoms such as nausea, vomiting, abdominal pain, indigestion, and as an antispasmodic. 
Aromatherapy can potentially treat many illnesses. Whether essential oils only work as placebos, or are actually medically-active is unknown at this point. Following the list of essential oils above, or other essential oils, you can potentially affect your mood using very low doses that are dissolved in fixed oils. If you develop rashes or other negative results be sure to stop.