There are several different methods of execution used by the states (as well as the federal government and the U.S. Military) that offer ways to carry out a sentence of death other than the lethal injection method.  

Image:  Florida’s Electric Chair

 

Guillotines, for example, are well known execution methods (as sadly are beheadings by other means), but the United States does not recognize this as an acceptable means of carrying out capital punishment.

Alternative Execution Methods

These are already in the law, and have been superseded by lethal injection as the preferred method of carrying out capital punishment.  They have passed constitutional challenge already, these methods just haven’t been used in decades.  But they’re available, statutorily.  

In these jurisdictions, lethal injection is considered the primary means of carrying out a death sentence, but other execution methods remain as acceptable alternatives in the state law.  
 
As lethal injections come under more and more scrutiny, these statutes are being reconsidered as ways to impose the death penalty and it’s probably going to be in the near future that these older methods may be used again.  
 
It may not take much more than an executive order from the governor (say, in Tennessee where the electric chair was restored by the governor in May 2014) for the state to opt for these alternative methods.  
 
4 Methods of Execution In U.S. Death Penalty Cases Other Than Lethal Injection
 
 
1.  Firing Squad
 
This month, the State of Utah made news by returning to the firing squad as an alternative, acceptable execution method to lethal injection.  However, this may not be a real surprise to those living in Utah; after all, the firing squad has been used as recently as 2010, when Utah law allowed a Death Row inmate to choose the firing squad over lethal injection as the method of execution.
 
 
Other states with firing squad as an approved method of execution:  Idaho and Oklahoma.
 
2.  Electrocution
 
States with electrocution (electric chair) as an execution method in their laws, while lethal injection became the preferred method of execution, are Alabama, Arkansas, Florida, Illinois, Kentucky, Oklahoma, South Carolina, Tennessee, and Virginia.
 
3.Gas Chamber
 
Gas chambers as a means of capital punishment exists in 5 states: Arizona, California, Maryland, Missouri, and Wyoming.  Oklahoma is currently legislating nitrogen in gas chambers as a means of execution, since the lethal injection method used in that state is being reviewed right now by the Supreme Court of the United States. 
 
4. Hanging
 
For New Hampshire and Washington, death sentences can be carried out by hanging as well as lethal injection.