As Terran, what techniques effectively harass in the early game (before 6 minutes)

Solution 1:

Just build multiple barracks with no addon buildings and pump out marines. Build offensive bunkers. You can put addons to the barracks and salvage the bunkers later to push into mid-game. An early reaper rush utilizes tech labs as well that can be transitioned into marauders later.

Your goal of course is to either walk your marines into the enemies mineral line and start shooting. They may pull their workers off or push to attack you, either way marines have a great advantage to just run back and shoot some more with a little micro. You can also take out pylons/depots/overlords with your marines to slow them down considerably by supply blocking them.

So either choose early reapers (which are rare these days) or mass marines and micro them carefully to avoid your enemies early melee defenders (you might have more trouble against Terran though, due to early walls and the fact they also have ranged units).

Solution 2:

Hellion harassment is a popular Terran strategy that can lead nicely into a 1:1:1 (Barracks, Factory, Starport) or mech-heavy build for the mid-game. Hellions are fast and kill workers very effectively. If any survive the initial harassment, they can be used for scouting, thanks to their speed.

You want to get your gas and factory both up quickly to facilitate this strategy. Many players will research the Infernal Preigniter upgrade as soon as it is available, as this greatly increases Hellions' ability to burn through the workers.

Inside the enemy base, you'll generally run the Hellions around behind the minerals, picking off workers while kiting any enemy defensive units.

You can also use this strategy against a walled-in opponent. Just drop the Hellions into the enemy base with Dropships. Just be aware that adding Dropships into the mix will mean your harass lands a bit later and will probably face stiffer defense. A walled-in player will probably be watching closer for drops as well.

Solution 3:

The Terran player qxc created an opening called Ghost First which is very good for early game harassment in TvT. The build is comprised of one ghost, one marine, and a few SCVs. The marine is used to back up the ghost and the SCVs are mainly used to distract and soak up damage. With good timing you can get everything out in less than six minutes. The basic idea is to impact you opponent's economy by using your Ghost to kill their SCVs.

Here's the goal (excerpted from link above):

The goal of the opening is to attempt to use the Ghost for a very early timing attack. It attacks the opponent early to try and harass the mineral line. The attack is not an all-in but is simply a way to exert pressure on your opponent. Pulling back your Ghost is recommended if they are defending. Following the attack, you can comfortably transition into a Marauder based Bio build and move into the mid-game.

The Good

This build is somewhat safe against many of Terran's early game units and does well against early aggression.

The Bad

This build requires good micro and control over the Ghost's abilities.