The history of urea synthesis is followed by a description of the fundamental thermodynamics underlying urea production. The basic chemistry, side reactions and physical chemistry are summarized, after which the way these fundamentals logically result in a number of constraints to urea production processes is explained. Challenges in urea production process design are highlighted. Finally, using a number of recent urea processes as examples, it is shown how these challenges are addressed in the industrial urea production processes currently offered by the main urea process licensors.