Electronic system-level (ESL) is a type of design engineering that uses computer software to create systems made of many interconnected parts. Instead of designing these parts one at a time, ESL makes it possible to design whole systems all at once. This makes it easier and faster to create complex systems like mobile phones, computers and industrial machines. ESL also makes it possible to test these systems in virtual reality, which means engineers can make sure a system works before it ever gets built.