Research

Research Tree

You are given Laser weapons, Medi-Kits, and Motion Scanners as research topics at the start of the game. Every subsequent research topic stems from a breakthrough in a previous research topic, or new technology picked up on the battlefield and brought back to your stores.

Tip: You can sell off the item from your stores once research has started, if you need the space. The research will continue (unless you cancel the project!). Also, you get the same amount of cash, whether you've researched it or not.

Laser Weapons

Laser Weapons  (tech breakthrough)
  |
Laser Pistol
  |
Laser Rifle
  |  
Heavy Laser       
  |  
Laser Cannon 
(Laser Tank)
  | 
Laser Defense

Plasma Weapons

    Heavy Plasma               Plasma Rifle 
         |    Heavy Plasma Clip   |  Plasma Rifle Clip  
         |___________|            |__________|
                 |                       |
                 |                       |
                 |__________OR___________|
                            |
                      Plasma Cannon
                   (Hovertank/Plasma) *See Note 1
                            |
                      Plasma Defense

Fusion Weapons

 Blaster Launcher    Blaster Bomb
             |___________|
                  |
               Fusion Ball
          (Hovertank/Launcher)  *See Note 2
                  |
          Fusion Ball Defense

Armor

      Alien Alloys
              |  
Elerium-115  Personal Armor  UFO Power Source      
|_____________|_________________| 
          | 
          |     
     Power Suit      UFO Navigation  
          |_________________|   
                   |
              Flying Suit 

Craft

Alien    Elerium        UFO            UFO
Alloys      115     Power Source   Navigation
|___________|___________|______________|
           |
   UFO Construction
           |
           |
   New Fighter Craft
      (Firestorm)  *See Note 1 (Required for Hovertank/Plasma)
           |
New Fighter-Transporter
      (Lightning)
 __________|____________
 |                      |
Grav Shield     Ultimate Craft (Avenger)

Notes

1. "New Fighter Craft" required before you can build the Plasma Hovertank.
2. Fusion Ball Launcher required before you can build the Launcher Hovertank.

-- from Kasey Chang's USG

Live Alien Research

Researching live aliens gives different results depending on their rank. It is very important to take certain aliens alive to finish the game. Picking your aliens becomes easier with the Mind Probe and Small Launcher, although a well-applied Stun Rod early in the game can give your tech tree a big boost. Note you not only need to have an unconscious alien when you exit the Battlescape, you also need an Alien Containment module at the base your craft returns to, or they'll die before you get any useful info.

• Any Live Alien
  • Alien Origins
• Alien Soldier
  • Nothing, beyond information on own species
• Alien Medic
  • Species information *1
• Alien Navigator
  • Alien missions (harvest, base, etc.)
  • Hyper-Wave Decoder

• Alien Engineer
  • UFO Types (they will show you all UFO types, not just their own)

• Alien Leader (or Commander)
  • The Martian Solution

• Alien Commander
  • Cydonia or Bust *2

• Any Ethereal, or Sectoid Leader/Commander
  • Psionics (see below)

Notes

1. Alien medics can give information on species other than their own. This is the only way to gain information about "live" Cyberdiscs. Information gained about Etherals (or Sectoids) in this way will not give you access to Psionics, however.
2. Commanders can be found in their Battleships, or their bases (or your bases, should they choose to attack). The USG incorrectly states that only a Commander from an alien base can be used to give access to "Cydonia or Bust", however, in practise any Commander will do - so long as The Martian Solution has already been researched. Should you research a Commander prior to this, you'll have to capture another.

Psionics

Psionic Alien
  |
Psi Lab
  |_______
  |       |
Psi-amp Mind Shield

Endgame

Any Live Alien
 |
Alien Origins
 |
 | Any Live Alien Leader
 | /
The Martian Solution  
 |
 | Any Live Alien Commander
 | /
Cydonia or Bust
 |
 | Avenger
 | /
Cydonia (mission)

-- from Kasey Chang's USG