Heat therapy, also called thermotherapy, is the use of heat in therapy, such as for pain relief and health. It can take the form of a hot cloth, hot water bottle, ultrasound, heating pad, hydrocollator packs, whirlpool baths, cordless FIR heat therapy wraps, and others. It can be beneficial to those with arthritis and stiff muscles and injuries to the deep tissue of the skin. Heat may be an effective self-care treatment for conditions like rheumatoid arthritis.
Heat therapy is most commonly used for rehabilitation purposes. The therapeutic effects of heat include increasing the extensibility of collagen tissues; decreasing joint stiffness; reducing pain; relieving muscle spasms; reducing inflammation, edema, and aids in the post acute phase of healing; and increasing blood flow. The increased blood flow to the affected area provides proteins, nutrients, and oxygen for better healing.
Heat creates higher tissue temperatures, which produces vasodilation that increases the supply of oxygen and nutrients and the elimination of carbon dioxide and metabolic waste.
Heat therapy is useful for muscle spasms, myalgia, fibromyalgia, contracture, bursitis.
Moist heat can be used on abscesses to help drain the abscess faster.  A study from 2005 showed heat therapy to be effective in treating leishmaniasis, a tropical parasitic skin infection. 
Heat therapy is also sometimes used in cancer treatment to augment the effect of chemotherapy or radiotherapy, but it is not enough to kill cancer cells on its own. 
Heat therapy is contraindicated in case of acute injury and bleeding disorders (because of vasodilation), tissues with a severe lack of sensitivity, scars and in tissues with inadequate vascular supply (because of increased metabolic rate and demand which a tissue with poor blood supply may fail to meet resulting in ischemia).
The use of Heat therapy for deep-seated tissue can be treated with shortwave, microwave, and ultrasonic waves. This produces a high temperature that penetrates deeper. Shortwave produces a 27MHz current, microwaves use 915 and 2456 MHz, and ultrasound is an acoustic vibration of 1MHz. The way ultrasonic waves work is they selectively superimpose the incoming wave and increase the energy for absorption, and the significant part of the longitudinal compression gets converted into shear waves. When they are rapidly absorbed, the interface between soft tissue and bone is selectively heated. 
Heat therapy can be used for the treatment of headaches and migraines.
Many people who suffer from chronic headaches also suffer from tight muscles in their neck and upper back. The application of constant heat to the back/upper back area can help to release the tension associated with headache pain. In order to achieve heat therapy for headaches, many use microwaveable pads which can often overheat, potentially leading to injury, and lose their heat after a few minutes.
Some new products use heated water, running through pads, to maintain a constant temperature, allowing headache sufferers to use hands-free heat therapy in the treatment of their headache pain.
Thermotherapy increase the extensibility of collagen tissues. Using heat, it can relieve the stiffness in joints in different cases. Shortwave and Microwave heat application may reduce muscle spasms, and selective heating with microwaves can accelerate absorption of hematomas.
This will, in turn, allow the stiff muscle to stretch. Ultrasounds are not absorbed significantly in homogenous muscle. Heat therapy using hyperthermia has been used to treat cancer in combination with ionizing radiation.