Topic must be beyond undestanding of average layperson, and would assits trier of fact to understand evidence. Expert must be qualified by knowledge, skill, training, education, experience.
FRYE: if opinion based on scientific theory, theory must be generally accepted in relvant community.
May answer hypotheticals or base opion on inadmissible evidence, if it's the type reasonably related upon in field.
Opinion on ultimate issue permitted if not incorporating legal opinion.