(Take a deep breath)
Ok, this is a very generic title, but for many projects this describes two broad areas:
Electronic and Mechanical Engineering. Of course each of these disciplines should be expanded or even broken-out completely.
For example: if there is one or more microcontrollers or microprocessors in your product it is important to consider software development. Even here: there is a distinction between "software" vs. "firmware" as they involve different mindsets and different tools.
Another example considers the various sub-disciplines in the mechanical engineering world, where expertise may be needed in kinematics (mechanisms), thermal, fatigue and structural analysis, optics (which is very similar to the electronic discipline of "fields")
Often times we are faced with challenges generally considered far outside these two broad disciplines. One such challenge was the RF-enabled Liquor Management Spout which required an inexpensive solution but also required integrated circuits on three different planes, ultra-low power circuits, energy-harvesting electronics, Radio Frequency, Wireless Networking and Fluid flow analysis and it had to be dishwasher safe! Separate boards and connectors to accommodate the various orientation planes in such a small and inexpensive product would be too large and add too-much to the cost.
In the below picture: the Engineering required not only RF, Electronic and Mechanical skills, but Origami (the ancient Japanese art of folded-paper sculpture) without which, the substantial Engineering would have little-to-no value!