A preliminary hearing is exactly what the name implies: a hearing held at an early stage of a criminal proceeding. A preliminary hearing is intended to prevent unjust incarceration of persons charged with a crime by giving the court an opportunity to evaluate the prosecution's evidence. The procedures that are followed in a preliminary hearing are specified by Pennsylvania's Rules of Criminal Procedure.
The sole function of a preliminary hearing is the court's determination of whether the prosecuting authority has a "prima facie" case that a crime indeed took place and that the particular crime was committed by the accused. A prima facie case is one in which the prosecution is able to produce sufficient evidence to convince the judge that the case should be tried and that the defendant should either be incarcerated until trial or released on bail.
A defendant has crucial rights at the preliminary hearing. These include the right to have a lawyer, to question witnesses called by the prosecutor and call witnesses for the limited purpose of rebutting the prosecution's evidence. The formal rules of evidence that govern trials are relaxed for a preliminary hearing. The court may consider any relevant evidence presented by the prosecution or the defendant, including hearsay. If the court finds that a prima facie case has been established, the defendant will either be returned to jail until the trial or released on bond. If the judge finds that the prosecution has failed to establish a prima facie case, the criminal charges will be dismissed and the defendant will be discharged.
It can be damaging to attend a preliminary hearing without counsel. An experienced criminal defense attorney will be able to evaluate the strength of the prosecution's case and advise the defendant about various options, including asking the prosecution to dismiss the charges if the case is especially weak.
Source: Pennsylvania Rules of Criminal Procedure, "Rule 542," accessed on Oct. 20, 2014