Thais are a polite people and, while remarkably tolerant of foreigners gallivanting on their beaches and with their women, you'll find that you will get more respect if you in turn treat them and their customs with respect.
The traditional greeting known as the wai, where you press your hands together as is in prayer and bow slightly, is derived from the Hindu cultural influence from India, and still widely practised. Among Thais, there are strict rules of hierarchy that dictate how and when the wai should be given. In brief, subordinates salute superiors first. You should not wai service people or street vendors. The higher your hands go, the more respectful you are. You will also often see Thais doing a wai as they walk past temples and spirit houses. As a foreign visitor, you are not expected to know how to wai, nor to reciprocate when wai'd to; while you're unlikely to cause offense if you do, you may well look slightly strange. If somebody makes a wai to you, a slight bow alone is more than sufficient for ordinary occasions, and for business, most Thais will shake hands with foreigners instead of waiing anyway.
Personal appearance is very important in Thailand as a measure of respect to other people, you will find that dressing appropriately means that you are shown more respect in return. This translates in many ways, even sometimes lowering initial offering prices at markets. While some allowance is made for the differing customs of foreigners, Thais respond more positively to well-dressed Westerners.
While the most touristy parts of Bangkok, Pattaya and Phuket are exceptions, traditionally, Thais are modest and conservative dressers. At a minimum your clothes should be neat, clean, and free from holes or tears. Except at the beach or at sacred sites normal western dress is acceptable for both men and women, except that you should avoid clothing showing a lot of skin. Long pants are preferable to shorts, you should always cover your shoulders and if you are a woman, you should not be exposing your cleavage. Outside of the major temples and royal palaces, men wearing shorts is generally acceptable. Both men and women should avoid being topless in public, as Thais consider this to be uncivilised. Although some leeway may be given at the most touristy beach resorts, it is technically illegal, and a male tourist was arrested in 2016 for going topless at Songkran celebrations in Chiang Mai.
Taking off one's shoes at temples and private homes is mandatory etiquette, and this may even be requested at some shops. Wear shoes that slip on and off easily. Flip-flops, hiking sandals, and clog-type shoes are usually a good pragmatic choice for traveling in Thailand; only in the most top-end establishments are shoes required.
It is best to play it safe with wats and other sacred sites in Thailand; your dress should be unambiguously modest and cover your entire torso and most of your limbs. For men, ankle-length pants are mandatory; on top, t-shirts are acceptable, though a button-front or polo shirt would be best. Many recommend that women wear only full length dresses and skirts; you should make sure that your clothing covers at least your shoulders and your knees and some places may require that you wear ankle-length pants or skirts and long sleeved tops. Shorts and sleeveless shirts are highly inappropriate, as are short skirts. The rules are even more strict for foreign visitors, so even if you see a local in shorts it's not OK for everyone.
Topless sunbathing is common by Western women at many tourist beaches. At beaches which are primarily Thai visitors, however, this is not advised.
Monks are an integral part of Buddhism in Thailand, and Thai men are generally expected to spend a certain amount of time living as a monk at least once in their lifetime.
Buddhist monks are meant to avoid sexual temptations, and in particular they do not touch women or take things from women's hands. Women should make every effort to make way for monks on the street and give them room so they do not have to make contact with you. Women should avoid offering anything to a monk with their hands. Objects or donations should be placed in front of a monk so he can pick it up, or place it on a special cloth he carries with him. Monks will sometimes be aided by a layman who will accept things from women merit-makers on their behalf.
Theravada Buddhist monks are also supposed to avoid material temptations and as such, are not allowed to touch money, so offering money to a monk is considered to be a sign of disrespect in most Theravada Buddhist cultures. Therefore, should you wish to donate to a monk, you should only offer food, and put your monetary donation in the appropriate donation box at the temple. Those monks that accept money are almost always fakes.
As in neighbouring countries, the swastika is widely used in Thailand as a Buddhist religious symbol. It pre-dates Nazism by 2,500 years and has no anti-Semitic connotations.
When entering temple buildings, always take off your shoes before you do so, as entering a temple with footwear is considered to be a major faux pas. When sitting on the floor in a temple, make sure you cross your legs under you "mermaid-style" so your feet do not point at any person or statue. Do not pose alongside a Buddha statue for a photo and certainly don't clamber on them. It's OK to take photos of a statue, but everyone should be facing it. Also, as doorway thresholds are considered a sanctuary for spirits, it's important not to step on a raised threshold, but rather to step over it.
Avoid wearing clothing or displaying tattoos with the Buddha or other Buddhist imagery, as it is considered to be very disrespectful.
A sizeable Muslim minority (about 5%) is also present, mainly concentrated in the southern provinces, but also with a significant community in Bangkok. Most Thai Muslims are ethnic Malays, though there are also substantial numbers of Muslims who are ethnically Thai, or descended from Indonesian, Cambodian Cham or South Asian immigrants.
It's illegal to show disrespect to royalty (lèse-majesté), a crime which carries up to 15 to 20 years imprisonment. Do not make any negative remarks, or any remarks which might be perceived as disrespectful about the King or any members of the Royal Family. Since the King is on the country's currency, don't burn, tear, or mutilate it - especially in the presence of other Thais. If you drop a coin or bill, do not step on it to stop it - this is very rude, since you are stomping on the picture of the King's head that is printed on the coin. Also, anything related to the stories and movies The King and I and Anna and the King is illegal to possess in Thailand. Almost all Thais, even those in other countries, feel very strongly when it comes to any version of this story. They feel that it makes a mockery of their age-old monarchy and is entirely inaccurate. In 2007, a Swiss man (Oliver Jufer) was sentenced to 10 years imprisonment for spraying graffiti on the King's portrait, although he later expressed remorse and was pardoned by His Majesty personally (quote: "It troubles me when such harsh sentences are passed.") and deported to Switzerland.
Elephants are a large part of Thailand's tourist business, and the smuggling and mistreatment of elephants for tourist attractions is a widespread practice. Be aware that elephants are often separated from their mothers at a young age to be cruelly trained under captivity for the rest of their lives. If you intend to go on an elephant ride, purchase an elephant painting, or "use" elephants for other activities, please take their mistreatment into account. There are a few ethical animal tourism operators in Thailand such as Elephant Nature Park in Chiang Mai.
A depressingly common sight on the congested streets of Bangkok and other tourist centers is elephant begging. During night hours, mahouts (trainers) with lumbering elephants approach tourists to feed the creatures bananas or take a photo with them for a fee. The elephants are brought to the city to beg in this way because they are out of work and are mistreated and visibly distressed under the conditions of the city. Please avoid supporting this cruelty by rejecting the mahouts as they offer you bananas to feed the elephants.
Drugged animals such as lizards and birds are sometimes used by touts as photo subjects. These touts are often seen plying the main tourist beaches of Thailand. The tout will take a photo with you and the doped up animal and then demand payment.
Rare and endangered species are often sold at markets for pets, and many other animal products are sold as luxury items. Avoid buying rare pets, leather, ivory, talons, dried sea creatures (such as starfish), fur, feathers, teeth, wool, and other products since they are most likely the result of illegal poaching, and buying them contributes greatly to animal endangerment and abuse.