In the first part of my talk, I plan to review several applications described by delay differential equations (DDEs) starting from familiar examples such as the control of the shower temperature and car following models to physiology and industrial problems. DDEs have the reputation to be mathematically difficult but there is a renewed interest for both old and new problems. In the second part of my talk, I’ll emphasize the need of analytical tools in order to guide our numerical simulations and identify key physical phenomena. These ideas will be illustrated by recent optoelectronic oscillator problems.