In Texas, it's illegal to operate a motor vehicle while you are intoxicated. To the police and prosecutors, "intoxicated" means one of two things:
1) A blood alcohol concentration (BAC) of .08 or greater
2) Not having normal of use of your mental or physical faculties due to the influence of alcohol, a controlled substance, a prescription drug, or any other substance. (Texas State Penal Code, § 49.0)
What does this mean to you as a driver? First of all, it means that there is no blood alcohol concentration under which it is guaranteed to be safe and legal to drive. Even if your BAC is under .08, you could still be charged and convicted of DWI if a police officer stops you and believes that your reflexes and/or judgment are impaired. Secondly, it means that even if you have a high enough alcohol tolerance to retain control of your mental and physical faculties at a BAC of .08 or higher, it's still illegal for you to drive in that condition.
There are other laws, too, that deal with drunk driving. For example, if you get behind the wheel of a vehicle while you are intoxicated and you have a child in the car with you, you can be charged with a felony in the state of Texas. If you crash your car while driving drunk and seriously injure another person, you've committed “intoxication assault," which is also a felony. If your drunk driving kills someone, you can expect to be charged with "intoxication manslaughter," a second-degree felony.