This book presents lectures by prestigious researchers and experienced observers from the eighteenth Winter School of the Canary Islands Astrophysics Institute (IAC), devoted to emission lines and the astrophysical objects that produce them. It shows how emission lines in different wavelengths, from ultraviolet to near infrared, can provide essential information on understanding the formation and evolution of astrophysical objects, from the first stars to objects in our Galaxy. It includes practical tutorials for data reduction, making this a truly valuable reference for researchers and graduate students.