This book discusses a group of natural compounds that is referred to in many bibliographic references for its multiple medical and therapeutic applications, which have been carried out by civilisations in the past and continue to be used in the present. Thus, the alkaloids have been isolated from marine and terrestrial sources and human beings have had the aptitude to determine the chemical structure of many derivatives of simple and big complexity as well as observing the biological effects of every compound in the living organism. Different natural sources as well as the synthesis of many alkaloids of big therapeutic activity have been the basis for the hundreds of drugs that are applied successfully in the scope of the health and combating diverse diseases. Alkaloids' low cytotoxicity in many cases and versatility in transforming into stable salt have generated diverse drugs of easy administration in the organism without the side effects associated with the ingestion of organic and inorganic salt of difficult tolerance. In this sense, this contribution covers several chapters which include: mechanisms and strategies against cancer, wherein certain types of alkaloid take control of important and selective form; the use of boldine as the alkaloid of current reference in the traditional medicine and used actively as natural antioxidant; alkaloids from vegetable origin as coming from the Amaryllidaceae; curious brominated alkaloids from marine sources between several outstanding examples; alkaloids derived from the Erythrina including the synthesis and pharmacological applications; the technological approaches of some derivatives originated from Tropane; an interesting contribution of the application of Trabectedin as alkaloid of clinical use in the treatment of ovarian cancer; the mention of a small group of alkaloids called oxoisoaporphines as the big medical tool in the treatment of mental disorders such as depression; and finally a complete review on the Daphniphyllum alkaloids.