[research tree](https://vignette.wikia.nocookie.net/xcom/images/7/72/Xcom-enemy-unknown-2012-tech-tree-research-chart.png/revision/latest?cb=20121026045427) Strategy: * [XCOM: Enemy Unknown guide - squad builds, ET tactics, base-building strategy and more](https://www.pcgamer.com/xcom-enemy-unknown-guide/2/) * [8 tips for saving the world in XCOM: Enemy Unknown](https://venturebeat.com/2012/10/17/xcom-enemy-unknown-game-guide-strategic/) * [lists of tips](https://steamcommunity.com/app/200510/discussions/0/405691491125693652/) Recommended upgrades, from the above strategy article: * **Heavy** class -- Burst of Steel -- Suppression -- H.E.A.T. rounds * **Assault** class -- Lightning Reflexes -- Close Quarters Fighting -- Rapid Fire * **Sniper** class -- Snap Shot -- Battle Scanner -- Opportunist * **Support** class -- Field Medic -- Revive -- Deep Pockets * **Psi ops** class -- Mindfray -- Psychic Inspiration -- Mind Control