In season 4's "Angels," Munch briefly goes undercover as a pedophile looking to book an illicit sex tour.
Also in season 4, "Risk," Stabler goes undercover to bust a cocaine ring.
In the season 7 premiere, "Demons," Stabler goes undercover as a pedophile.
In season 7's "Starved," Benson goes undercover at a speed-dating service.
In season 8's premiere, "Informed," Benson gets pulled into an FBI undercover op with a suspected eco-terrorist group. This led into Mariska Hargitay's maternity leave. The storyline was picked up when she came back six episodes later, in "Infiltrated."
In season 9's "Undercover," Benson goes under as a prisoner to catch a rapist guard.
In season 10's "Wildlife" Stabler goes under to investigate a smuggling ring.
In season 10's "Crush," the whole squad fakes a trial to catch a judge abusing her power.
In season 11's "Beef," Benson goes undercover at a meat-packing plant.
In season 11's "Ace," Benson and Stabler pose as a married couple to bust an illegal adoption ring.
In season 12's "Mask," Stabler goes under as a sex addict.
In season 12's "Bombshell," Benson and Stabler go under as married swingers.