When it comes to spa treatments, the answer to the question of whether or not you should take your clothes off is not a simple one. It depends on the type of treatment you are receiving and the spa's policy. Generally speaking, for massages and body treatments, it is recommended that you undress completely, including your underwear. Most spas provide paper panties for modesty during body treatments such as scrubs, wraps, or self-tanners.
For other treatments such as Reiki and Shiatsu, it is usually done through clothing. When you arrive at the spa, the receptionist will likely ask you to put on a gown. How much should you undress? It is recommended that you keep your underwear on. If you are having a back massage, it is best to take off your bra as it can get in the way of the massage and make it difficult to relax.
You can also leave your socks on if you are only having an upper body massage. You don't need to bring a robe and slippers to the spa as most spas will have them ready for you when you arrive. Depending on the treatments you receive during your visit, you can also change to a new gown if it gets dirty or damp. The same goes for towels; your spa must provide them for you so you don't have to bring your own.
In addition, some spas may also offer other services such as showers and toiletries to use before and after your visit.