Those of us that are old enough recall the computer revolution

 These include microprocessors, remote telecommunications, Selective Availability Directive, and the internet.
Those of us that are old enough recall the computer revolution of the early 1980's where monochrome screens replaced the traditional typewriter and limited software was an incredible breakthrough. Since that time, computers have become remarkably powerful and part of our daily lives. Microprocessors incorporate the function of a computer's Central Processing Unit (CPU) onto a single integrated circuit. GPS tracking systems use microprocessor technology to incorporate the GPS receiver, telecommunications module, power supply, and any additional add-on modules. These Printed Circuit Boards (PCB) provided the platform and conductive pathways to connect these modules creating the mobile GPS tracking device. In the 1980's, PCB's and microprocessors were cost-prohibitive with limited performance capabilities. GPS tracking devices were limited in the market not by its ingenuity but by the technological and cost limitations. Very few manufacturers built tracking devices in the 1990's due to these limitations. However, those who did and still manufacture them to this day should be commended for having the foresight and resolve to fight through these challenges. Since that time, advancements in digital technology has reduced the size and lowered the cost to manufacture these devices.
The most important thing about a good mobile tracking device is that they can allow you to see everything that's happening on the phone. Whether it's the text messages, phone calls, contact information, browser history, and everything like that - you want to see every single thing that's happening. GPS information is very important, too, and you should aim to get a good tracker that allows you to see, live, where the phone currently is.
When you say device it's a bit of gray area. To narrow it down, we should only focus on software-based devices. For instance, if you use a physical device, you run the risk of the phone being shaken and knocking out the tracking product. You also run the risk of the person looking at their phone and seeing the chip, which is something that is unacceptable. You should only use software.
Warning: Don't download anything that isn't trusted. If you don't see a lot of good reviews, don't download it. Since we are going with software this is the only risk (it might be a virus.) This is very easily avoided, though, by checking reputation and reviews.