What are the Basic Rules of Evidence for Testifying as an Expert?

Federal Rule of Evidence 702
Testimony by Experts

[If] Testimony providing scientific, technical or other specialized [knowledge] information, in the form of an opinion or otherwise, may be permitted only if (1) the information is reasonably reliable and will substantially assist the trier of fact to understand the evidence or to determine a fact in issue, and (2) the [a] witness is qualified as an expert by knowledge, skill, experience, training, or education to provide such testimony, [may testify thereto in the form of an opinion or otherwise.] Except with the leave of court for good cause shown, the witness shall not testify on direct examination in any civil action to any opinion or inference, or reason or basis therefore, that has not been seasonably disclosed as required by Rules 26(a)(2) and 26(e)(1) of the Federal Rules of Civil Procedure. (Italicized portions indicate 1993 text; bracketed text indicates old version.)

Federal Rule of Evidence 703
Bases of Opinion Testimony by Experts

The facts or data in the particular case upon which an expert bases an opinion or inference may be those perceived by or made known to the expert at or before the hearing. If of a type reasonably relied upon by experts in the particular field in forming opinions or inferences upon the subject, the facts or data need not be admissible in evidence.