For an expert to be permitted to testify first they must qualify as an expert. Then the experts opinions will be permitted only when

1). The opinions assist a jury in understanding questions that inexperienced people are not likely to decide correctly without the assistance of an expert. In circumstances where knowledge, training, and experience enable the expert witness to form an opinion that is going to aid the jury then an expert witness will be permitted to offer his testimony.

2). When the opinions are based on facts in evidence. The facts upon which the opinion of an expert witness is predicated must be stated. The jury must be informed of the facts or the assumed facts upon which the expert's opinion is based. An expert witness may not deliver his opinions in a factual vacuum. Furthermore, there must be evidence to support these facts. Beyond that it is for the jury to determine whether the facts exist or not. It has been the practice in Maryland for some years to permit an expert to express his opinion upon the facts in evidence which he has heard or read on the assumption that these facts are true. Twombley v. Fuller Brush Co., 221 Md. 476, 158 A.2d 110 (1960). Although expert opinion that assumes the truthfulness of disputed testimony is generally admissible, the expert opinion that asserts that the disputed testimony is true is not admissible. Hall v. State, 107 Md. App. 684, 670 A.2d 962 (1996. Md. Rule 5-703 If the jury's finding on controverted questions of fact is contra to the premise upon which the expert bases his opinion, his opinion falls with the premise. Mehlman v. Powell, 281 Md. 269, 378 A.2d 1121 (1977).

Experts Use Of Opinions as a Basis For Opinions.: It is generally true that the opinion of an expert may not be based in whole or in part on the conclusions and opinions of other witnesses. Jackson v. Jackson, 249 Md. 170, 238 A.2d 852 (1968); nor on reports of others if they contain only opinions, inferences or conclusions. Pennsylvania Threshermen and Farmers' Mut. Cas. Co. v. Messenger, 181 Md. 295, 29 A.2d 653 (1943), Experts may rely on opinion evidence, based in part on reports of others which are not in evidence, but which the expert customarily relies on in the practice of his profession. Cohen v. Rubin, 55 Md. App. 83, 460 A.2d 1046 (1983).Where an expert relies on reports of others, he must demonstrate to the court not only that the reports were made in a reliable manner, but that they are reliable sources of information for the purposes to which the expert puts them. Madden v. Mercantile-Safe Deposit & Trust Co., 27Md. App. 17, 339 A.2d 340(1975).
 

Never under estimate the use of a Motion in Limine to keep the other parties expert from testifying in your case. I have won several cases using exactly this tactic to exclude the defendant's experts.. I first review the discovery disclosure to be precisely sure what opinions the expert is being offered to render. Each opinion must be based on fact, I want those facts disclosed in discovery. Next I analyze the benefit to the jury from the use of this expert. Most times defendant's prefer to use experts as window dressing and the expert offers nothing to the understanding and benefit of the jury. These figure heads can not be permitted to testify. You will many judges will agree.