Every Unit and Weapon has its own technological tree (see the diagrams to the right). You must research a unit before you can start building (or buy) it. To do that, you must research the unit's type, and then all of the unit's classes. Certain classes can only be researched when another class is researched, and all classes require a specific type to be researched. There is a certain cost of researching, and it takes a few minutes to research a type, from half an hour to two hours to research a class, and up to five hours to research a specific unit. Units will only appear on the tech tree when at least one of their type is researched.
The next part of researching involves modules and hulls - the parts of a unit. All units have a tech tree with every module that can be equipped onto them, and usually at least one extra unit. You need to research one module to get the next one. For example, in the above diagram of a branched tech tree, you would need the ejection seat researched to research the F-16 (along with the missile) or F/A-18. All modules fall under categories, like missiles, bombs, and seekers. Some fall into subcategories. Once you research a category, you can research anything in the category, provided that it doesn't fall into an unresearched subcategory, or research a subcategory in it.
Players need more than just money and patience in order to build hulls and modules - they also need materials. Units drop materials when destroyed, and you need only to stay next to the destroyed unit for a few seconds to collect them.
Like in real life, materials also fall into categories, like wood, and subcategories, like oak and steel.
Some AI-controlled Units at specific bases on some maps will also collect materials that they will deposit at the base. These materials can be collected at any time during the battle. The units will stop collecting and giving away free materials when destroyed, at which point the unit's materials can be salvaged.
You will not suffer in any way when your unit has its materials collected by someone else.
Some units and weapons can only be unlocked by completing Challenges. In this case, the item will immediately be given.