Little Jimmy hasn't been pulling his weight around the office lately. His addiction to bath salts has caused him to miss work, verbally assault co-workers, and beat an old woman with a shovel. You've decided to let him go. But can he be fired "at-will" or only "for cause"?
Generally, most employees are employed at will. This means you can fire them for any legal reason, such as incompetence, insubordination, or inception (if they are haunting your dreams.) What you can't fire them for is race, religion, age, or disability. At will employment status is generally the default for all employees absent an employment agreement that provides otherwise. Nearly all hourly-wage employees are employed at will.
On the other hand, if there is an employment contract, the employee is probably not at will. Instead, he can only be fired for cause. What is a proper cause for termination? Common examples include theft, insubordination, or conduct violations such as mooning your boss or sexually harassing a coworker. Check the employee's contract or the company's employment manual as well; often "for cause" is defined in these documents.
If you are unsure whether or not an employee can be fired legally, the best route to take is to consult an attorney beforehand. Generally, if the employee is not working under a contract, you can fire him for any reason. If he has a contract or tenure, things become much more difficult.
When it comes to Jimmy, the bath salts-addicted employee, if he is determined to be at will, fire away! If he has a contract, the missed work, co-worker harassment, and pending felony charges would all probably qualify as independent proper cause for termination.