"Low-level laser therapy (or photobiomodulation therapy) is a rapidly growing approach to treating a wide range of diseases and disorders that afflict humanity. This Tutorial Text covers the basic molecular and cellular mechanisms of action, applications for treating diseases in animal models, and its use in clinical trials and therapeutic practice in patients. Other topics include the two basic chromophores and how they trigger the signaling pathways, activation of transcription factors, and mobilization of stem cells; how the light-source design and the relevant energy parameters can affect the outcome of therapy; and the physics and tissue-optics principles that concern LLLT"--.