Pennsylvania law permits either party to introduce the use of expert testimony during trial, so long as the party complies with the rules of evidence. The party must first assert that an expert is needed to give the type of testimony and evidence sought to be admitted. Stated in other words, a court will only allow the testimony of an expert witness, if the testimony calls for scientific, technical, or other special knowledge, which a laywitness would be unable to testify. Therefore, the party must show that there is an issue at trial that can only be resolved by that expert witness' testimony.
Once the party seeking to use the expert witness overcomes that hurdle, the next step is qualifying the witness as an "expert." The party must establish that a particular witness has received specialized education, training, experience, etc. that would give this witness more knowledge than the average person in this field. Lastly, the party must establish that any tests the expert has used are "generally accepted" in his field of expertise.
In criminal cases, experts are often used in either violent crimes, such as assaults or murders, to give medical testimony regarding injuries; gun experts in cases dealing with firearms; or drug experts.