Can an expert witness file a declaration? Or do they have to testify in court?
The general rule is that a witness must appear and be subject to cross-examination. A declaration would in all likelihood be excluded as hearsay. I have tried cases before judges (not jury trials) where the judge permits the declaration filed under the penalties of perjury to serve in lieu of direct examination to conserve time but the expert must be made available for cross-examination.
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If you are talking about trial, then as previous answers have suggested, it is unlikely that a declaration would be sufficient. But there are other circumstances where expert declarations are allowed, such as on a summary judgment motion. If you are dealing with an expert declaration in that context, you might need to take the expert's deposition and offer some transcript excerpts in opposition or you might need to hire your own expert to raise an issue of fact.