Note: As you start X-Com 1, you will be asked what difficulty level you want to play. This makes the aliens smarter and they build more bases. This doesn't really matter, as when you save and then reload, it sets itself back to difficulty level 1 (beginner). I'm not sure if they fixed this in version 1.4, but in all earlier versions, just pick beginner level or play without reloading. You can also fix this by getting X-ComUtil. See the Downloads section for more details.

X-Com Base Building and etc...

When you start the game, you will be prompted to place your first base. You will want to set the first base in a good location, most likely in the middle of a large country. I suggest setting your base in the middle of Russia or in the middle of North America. The first base will have: 3 Hangers, 1 Living Quarter, 1 Laboratory, 1 Workshop, 1 Short Range Radar, 1 Storage Area, and 1 Entryway. The first things you want to build in your base are a Long Range Radar and more storage areas. It also is a good idea to build an Alien Containment to store any live aliens that might capture.

Start your research immediately! A good subject to start your research with would be Laser Weapons in X-Com 1 or Gauss Weaponry in TFTD. Medical Kits are also a must for research. As you collect alien technologies, first research Alien Alloys and the alien corpses. Make sure to use stun rods and stun bombs to capture live aliens. Certain technologies can only be researched after alien corpses and/or live aliens have been studied. Whenever a new live alien or corpse type comes into a base, research it immediately to get the most technology options opened for research. Research the alien weaponry first, then later study the alien ship technology.

Make sure to purchase more soldiers and scientists often. You will want at least 20 scientists before the first month is over. Getting your research going faster will help you keep ahead of those crafty aliens. More Living quarters will have to be built to hold all of the new troops/scientists/engineers, so start building a second living Quarters soon after the game starts.

Suggestion: Try to place your bases on opposite sides of the world. So, if you put Base One in Russia, put Base Two in America. You will want to place your bases so you can track UFOs as they travel, so if all your bases are clumped together, it will be of no use to your forces. It takes 10 minutes for each radar/sonar dish to make one complete rotation, so putting in multiple short range or long range radars will increase your UFO/SUB detection rate.

When building your base, you'll want to isolate the Hangars and Entry Bay from other structures. Aliens enter through these structures, so if you isolate them, you can easily defeat the aliens if your base is attacked. You'll want to make sure to build Living Quarters in all of your bases, and keep soldiers in them. Even if a base is being used only for research or interception, you need soldiers (and weapons) in it or if it is attacked it will be lost automatically.

Interception...

Using interceptors (and later using your own UFOs) is the biggest part of the game besides combat. You will want to outfit your interceptors with the latest technologies ASAP. At the start of the game, you will want to equip your interceptors with missiles, and later with alien plasma or blaster bomb technology. Until you have powerful technologies (i.e. Heavy Plasma blasters) send your interceptor only Very Small, Small,. or Medium UFOs. You don't want to tangle with Large UFOs just yet, so keep an eye on them. Only intercept Very Large UFOs with your own manufactured UFO craft. The normal interceptor can be toasted in two or three hits by a Very Large UFO. Another thing to remember if you are attacking larger UFOs is that you can send multiple interceptors. To make multiple interceptors attack at the same time, wait for one to catch up to the UFO. Click the "Standoff" button and minimize the window. Send out or order your other interceptors to attack and wait for them to catch up to the UFO. When the other interceptor's window pops up, click the Minimized Interceptor 1 icon and you will have two interceptors attacking. You can have a maximum of 4 interceptors attacking a UFO/SUB simultaneously. Try to use "Cautious Attack" or "Normal Attack" modes so that your interceptors don't get too damaged.
 

UFO Size/Usage Ratios

	Very Small	Small	Medium	Large	Very Large
UFO Defense	New Base  area Scouting	General Research  and Scouting	Research	City Attacks	Attack on X-Com bases
TFTD	New base  area scouting	General research  and scouting	Research	Island Attacks	Attack on X-Com bases

UFO Size/Alien Ratios

	Very Small	Small	Medium	Large	Very Large
UFO Defense	1 alien	3-5	6-10	10-15	20+ aliens
TFTD	1 alien	3-5	6-10	10-15	20+ aliens

General Combat Strategies for recovering downed UFOs...

When your transport/sub lands, you will want to move your troops out as soon as possible. It is a good idea to throw a Smoke/Dye grenade near the exit of your transport so aliens cannot see you exiting your ship and take shots at you. I recommend that you deploy your squads in groups of two. Send these groups out and spread them out as soon as possible. One good thing to do, for example, is have a group traveling with another group of two watching their back. You'll want to get someone to high ground in order to detect and neutralize aliens ASAP.

Suggestion: Save your game (in another slot, not on your regular game slot) at the beginning of each turn. If you lose any troops, abort the mission and reload from the saved game. This way will make the game easier as your forces will speedily increase in rank.

Your first priority in downed UFO/SUB missions is to find the UFO or SUB. Send your groups of two/three in different directions. Make sure to have a complete field of vision to prevent unknown alien attacks. Special precaution should be taken on missions with Chryssalids.  Once the UFO/SUB is found, be extremely cautious while approaching and entering it. There can be from 1-20 aliens in a UFO, depending on its size.

Note: Medium, Large, and Very Large UFOs have multiple floors. Travel between floors is done via teleporter/elevators. These are red squares on the floor.

General Combat Strategies for City Attack missions and Apocalypse missions...

In City attack missions, the rules are changed a bit. There is no UFO/SUB to recover, instead your mission is to eliminate any alien opposition with minimal losses of civilian lives. Again, moving your forces out in squads of two/three is a wise idea. Have squads check the interior of the buildings while one or two different squads cover them from the outside. As always, make sure to have a clear and complete view of the battlefield.

Advanced Combat Strategies...

You will want to put some of your troops in sniping positions. You should take 1 or 2 troops up stairs on top of buildings or use flying suits to get a large view of your surroundings. Check and recheck areas you have searched. Sometimes aliens will travel into areas you've been in and shoot you from behind. Be sure to research armors ASAP. You will want to get flying suits so you can get a clear view of the area.

In general situations, such as crossing a large open area with no cover, it is better to keep your units on Aggressive mode. Often in a combat situation, Cautious mode will cause your units to find cover rather than shoot an alien directly in front of them. This can be a major problem if they are in the open as they will run far away and have to be put back into position, wasting valuable TUs. However, if your units are outnumbered, outclassed, or are entering an area that has not been explored yet, Cautious mode or even Safe mode is good.

In Turn based gameplay, you will want to control your units in the same manner as in turn based mode. It is best to divide your troops into groups of 2 or 3, in that smaller groups will have less difficultly moving and will be able to get clear lines of fire.

Good luck!