In strategy computer games, the technology tree or tech tree is a hierarchical visual representation of the possible sequences of upgrades a player can take, by means of research. The diagram is tree-shaped in the sense that it branches at certain intervals, allowing the player to choose one sequence or another. Typically, at the beginning of a session of a strategy game, a player may only have a few options for technologies to research. Each technology that a player researches will open up more options, but may or may not, depending on the computer game the player is playing, close off the paths to other options. The tech tree is the representation of all possible paths of research a player can take.
A player who is engaged in research activities is said to be "teching up," "going up the tech tree," or "moving up the tech tree." Analysis of a tech tree can lead players to memorize and use specific build orders.
Types of tech tree 
Prerequisites for technology advances 
In many real-time strategy (RTS) games, the player needs particular buildings in order to research specific techs or build specific advanced units (StarCraft, Age of Empires, Empire Earth, Total Annihilation). In many turn-based strategy (TBS) games the prerequisite is one or more lower-level technologies, with no dependency on specific buildings (Master of Orion series, Civilization series, Space Empires series). Most strategy games however use both systems. both requiring dedicated buildings and in advanced cases pre requisite technology, sometimes culminating in a game ending super-weapon of some kind.
The structures of tech trees vary quite widely. In the simplest cases (e.g. Master of Orion) there are several completely separate research areas and one could research all the way up to the highest level in one area without researching other areas (although this would often be suicidal). In the most complex cases (e.g. Civilization) every technology above the starting level has more than one prerequisite and one has to research most of the lower-level technologies in order to research any of the top-level technologies. And there are many possibilities between these two extremes, for example in Space Empires researching to a specified level in one field may enable the player both to research to a higher level in that field and to start research in a new field which was previously not available.
Are all technologies available? 
Some RTSs make different techs available to different races or cultures (especially StarCraft; but many RTSs have special units or buildings for different cultures, e.g. Age of Empires expansion pack and later versions, Red Alert 2). Most TBSs make all technologies available to all cultures (e.g. Civilization). Master of Orion (original version) is a complex special case in this respect: the full tree is the same for all; but in each game each player gets a subset of the full tech tree that depends on which race was selected.
Balance between civilian and military techs 
In many RTS games tech advances are almost exclusively military (e.g. StarCraft). But in most TBS and some RTS games the research and production costs of top-end military techs are so high that you have to build up your economy and your research productivity first (RTS - Age of Empires and Empire Earth, where one of the most significant costs is going up an epoch; TBS - the Civilization series and Master of Orion series).
What happens after researching everything 
In many games there's nothing useful to do and the player may scrap research centers to save maintenance costs and/or devote the resources to something else (Space Empires series).
In later instalments of the Civilization series the last technology (called "future tech") represents an amalgamation of all possible future discoveries and can be researched repeatedly. In Civilization V, it increases a player's score, while in Civilization IV it raises the health and the happiness in the empire. Note that to reach the last technology, all spaceship technologies required to win must also have been discovered, so the game will be nearing its conclusion.
In the Galactic Civilizations series the final technology solves the nature of existence, and is victory.
In the Master of Orion series more advanced research reduces the size and cost of spaceship components, and "hyper-advanced" research in areas which have military applications therefore enables players to build more high-tech weapons into a given ship size and at lower production cost.
In Rise of Nations, the final four technologies result in such an advantage that the game will likely end quickly. Also, the "knowledge" resource needed to research is also used late in the game to produce cruise missiles and nuclear weapons.
The arcade shoot 'em up Gradius (video game) used a power-up system functionally identical to a tech tree in 1985.
Tech trees started showing up in turn-based strategy games around 1990, where Mega Lo Mania had a system of research levels/epochs that allowed the deployment of better units and defenses. Civilization (1991) was probably the first game to feature the same basic structure of tech trees seen in games today. 1992's Dune II is another example of an early game featuring tech trees.
See also 
- Rollings, Andrew; Ernest Adams (2006). Fundamentals of Game Design. Prentice Hall.
- See the relevant games' manuals.
- Master of Orion (1993), the Galactic Civilizations series and Sword of the Stars have no research buildings; players simply allocate some of each colony's output to research (see game manuals). In the Civilization series (1991 onwards) and Master of Orion II (1996) one can do research without buildings, but it's much faster when supported by the right buildings (see game manuals). In the Space Empires series (1993 onwards) and in Ascendancy (1995) research can only be done via buildings, but these can research any technology (see game manuals).
- "RTS Basics: R & D". and the StarCraft manual. Although multi-epoch games like the Age of Empires and Empire Earth series have a larger number of research options and a significant proportion of civilian research options, the research options all depend on having the right buildings. See the relevant games' manuals.
- "Joystiq interview: Ironclad talks 4X strategy with Sins of a Solar Empire". February 1, 2008. Retrieved 2008-06-14.
- In Warcraft III one can reach the highest level of one branch of the technology tree in five steps; Master of Orion (original version) has 10 levels per subject, and 2 to 5 technologies per level; the Civilization IV technology tree requires nearly 60 steps to reach the end (see game manuals).