A person or business that causes pollution or commits another type of environmental crime can be prosecuted. This involves the regulator bringing a case to trial in a criminal court.
The person or business charged with the offence (the 'defendant' in England/Wales, the 'accused' in Scotland) will usually be represented by a lawyer. Defendants/accused and witnesses may give evidence and be cross-examined. There are lots of rules about the procedures in criminal hearings, in order to ensure that the defendant or accused receives a fair trial.
Instead of prosecuting or recommending prosecution, regulators can deal with the problem through informal discussions with the polluter, perhaps issuing a warning or (in England/Wales only) a formal caution.
An alternative approach is for regulators to issue notices requiring a person to deal with a pollution problem. See the civil sanctions page for more details.