Fluorescence is a phenomenon that has existed for millions of years in the form of bioluminescence of fireflies, glow worms and bioluminescent sea creatures, and the northern lights. A review of references to luminescence in ancient manuscripts in various cultures all over the world through the ages can be found in Harvey’s (1957) detailed and comprehensive book, A History of Luminescence from the Earliest Times until 1900, as well as other historical studies on this subject (Lee 2008; Valeur and Berberan-Santos 2011) (Figure 2.1). The oldest account of what we now know to be fluorescence is said to date from 1565 when Nicolas Monardes observed and gave an account of a bluish glow in a cup made from wood used for medicinal purposes, lignum nephriticum (Acuña and Amat-Guerri 2008; Acuña et al. 2009). Observations with similar “glowing” features were made in 1646 by Athanasius Kircher and also later in the 18th and 19th centuries by others, for example, Boyle, Newton, and Brewster.