TvZ build order and strategy: Terran vs Zerg

Checklist: do this

Open with one barracks and expand early. Zerg always expands first, so you must too.
Academy by 3:30: stim, marine range, medics, and a comsat station for detection anywhere.
Add barracks until money stays near zero. Three by 5:00, five by 8:00, more on three bases.
Engineering bay by 5:00: +1 weapons, then +1 armor, and never let the upgrades stop.
A factory with siege mode by 7:00. Tanks exist to kill lurkers from outside their range.
Starport, then science facility, then two science vessels by 10:00 for detection and irradiate.
Third base around 9:00. The army moves out when stim, +1/+1 and the first vessels are done.
Scout at 2:30 with an SCV and at 6:00 with a scan, and answer what you see (below).

Why it works

Marines with medics are the cheapest army in the game and beat everything Zerg makes early; with stim, range and upgrades they stay good all game. Zerg's answers to them, lurkers and defilers, are answered in turn by siege tanks and science vessels. So the plan is simple: expand early, spend everything through many barracks, and let tanks and vessels handle the two things bio cannot.

If you remember one sentenceTake two bases, build more barracks than feels reasonable, keep marines and medics streaming, and make siege tanks the moment you see lurkers.

The build

Supply9 Time1:05 Supply Depot Never be blocked at 9 or 10. It is the first habit.
Supply11 Time1:35 Barracks One marine for safety and the academy later.
Supply12 Time2:00 Refinery Gas for the academy, stim and the factory.
Supply16 Time2:55 Command Center at the natural Two bases of income is what pays for five barracks.
Supply16 Time3:15 Supply Depot Ahead of the cap, always.
Supply18 Time3:35 Academy Stim, medics, marine range and the comsat all come from here.
Supply20 Time4:20 Comsat Station and Barracks 2 Detection anywhere; production must start growing now.
Supply24 Time4:45 Engineering Bay +1 weapons first; upgrades matter most for an army of marines.
Supply28 Time5:20 Barracks 3 and a second Refinery Three barracks keep two bases of minerals moving.
Supply32 Time6:00 Factory with Machine Shop Siege mode, then tanks, before lurkers arrive around 8:00.
Supply40 Time7:00 Barracks 4 and 5 Five barracks is the two-base standard.
Supply46 Time7:40 Starport, then Control Tower The road to science vessels.
Supply52 Time8:30 Science Facility Vessels, irradiate and +2/+2 need it.
Supply60 Time9:00 Third Command Center Three bases pay for eight barracks and constant upgrades.

Research, in this order

Stim Packs at the Academy
Marines and medics fight at half again their speed and rate of fire. Nothing else comes close.
U-238 Shells at the Academy
Marine range. Hydralisks and lurkers are fought at range.
Comsat Station at the Command Center
A scan reveals lurkers and shows you their base. Keep one energy in reserve.
Infantry Weapons +1 at the Engineering Bay
Marines kill zerglings in one fewer shot.
Siege Mode at the Machine Shop
Sieged tanks outrange lurkers and splash hydralisks.
Infantry Armor +1 at the Engineering Bay
Zergling and hydralisk damage drops by a step.
Irradiate at the Science Facility
Kills defilers, lurkers and mutalisk stacks.
Infantry +2/+2 at the Engineering Bay
Needs the science facility. Keep both going all game.

The army, stage by stage

Opening
Marine
One or two marines at home until the academy finishes.
Two bases
MarineMedic
About four marines per medic. Add a few firebats against mass zerglings.
Mid game
MarineMedicSiege Tank
Four to six tanks with the army, sieged before you fight lurkers.
Late game
MarineMedicSiege TankScience Vessel
Two vessels with the army, more later; irradiate defilers first.

If you scout…

A spire (mutalisks coming)

When 5:30 to 7:00, by scan or a scouting SCV

Do Missile turrets at every mineral line, keep marines in one ball with medics, add valkyries if the mutalisk count keeps growing.

Why Mutalisks kill workers and scattered marines; they lose to grouped stimmed marines under turrets.

A hydralisk den and no spire

When 5:30 to 7:00

Do Siege tanks with the army, siege mode done, and a comsat scan or vessel beside them. Do not stim into lurkers.

Why Lurkers kill marines that cannot see them; tanks outrange lurkers; detection lets everything shoot.

A spawning pool before 1:30, zerglings at your ramp by 2:40

When Your first SCV scout

Do Wall the ramp or bunker the natural, pull SCVs to repair, delay the expansion a little.

Why Twelve zerglings beat two marines; a bunker with repair beats twelve zerglings.

A third hatchery before any tech

When 4:00 to 6:00

Do Take your third command center sooner and attack at 7:30 with stim and +1.

Why Greedy Zerg has few units then; a timing attack punishes it, and your economy keeps pace.

A hive, defiler mound

When 12:00 onward

Do Vessels first, irradiate defilers on sight, keep tanks sieged behind the bio, never stim into dark swarm.

Why Defilers make bio useless under swarm; only tanks and irradiate work through it.

Counters

They have	You answer with
Zerglings	Marines and medics with stim; firebats Zerglings die to splash and to focus; medics keep marines alive through the surround.
Hydralisks	Marines with range and +1, tanks behind Hydralisks trade evenly with marines until upgrades and tanks tip it.
Lurkers	Siege tanks in siege mode with detection Tanks outrange lurkers; without detection nothing can shoot them.
Mutalisks	Grouped marines, turrets at mineral lines, valkyries Mutalisks lose to stimmed marines under turret cover and cannot fight valkyrie splash.
Defilers	Irradiate, then fight; tanks through swarm Dark swarm stops ranged bio; irradiate kills the caster.
Ultralisks	+2/+3 marines with medics, tanks, firebats Ultralisks soak damage; upgrades and numbers wear them down.

The mistakes that lose this matchup

The bank climbs
The sign Over 1,000 minerals after minute 8 with everything producing.
The fix Add barracks, two at a time. Money in the bank is army that does not exist.
Supply-blocked
The sign At the cap with money, twice a game.
The fix Depots when 10 supply from the cap; two at a time after minute 10.
One base too long
The sign No natural by 4:00.
The fix Expand at 16 supply after the first marine; Zerg is already on two hatcheries.
Marines into lurkers
The sign An army lost with kills in single digits.
The fix Tanks sieged first, detection beside them, then stim.
Never scouting
The sign Surprised by mutalisks or lurkers.
The fix SCV at 2:30, scan at 6:00; read the fork table.

By level

Newcomer

Learn the macro cycle before anything else: every 20 seconds, SCVs, marines, a depot if you are 10 from the cap, a barracks if you have 500 minerals. Get the academy at 3:30 and stim first. Make tanks when you see lurkers. Ignore every other unit and upgrade for now.

Learner

Hit the timings above within thirty seconds, scout at 2:30 and 6:00, and act on the fork table. Keep +1/+1 going from 5:00. Move out for the first time around 9:30 with stim, +1 and vessels, and take the third base behind it.

Competitor

Drop play into the main while the army pressures a third base, vessel control against scourge, tank lines that hold a third against lurker and hydralisk, and the defiler war: irradiate first, science vessels in pairs, and a +3/+3 army by 16:00.