Lay witnesses, are those who may have seen an injurious event occur or otherwise have personal knowledge of the event.  Typically, they cannot give their opinions about what they saw or heard.  They can only relay events as they perceived them occur.  An expert witness has not witnessed the event but has specialized knowledge relevant to it.  For example, a qualified mechanical engineer may be questioned in court as to whether he or she thinks a component in a crashed vehicle was defective.  Despite being able to offer an opinion, an expert witness’s testimony is subject to tight scrutiny and standards.