In many criminal cases, an expert report can make the difference between an acquittal and a conviction. A person may be regarded as an expert if they possess specific or specialised knowledge in a particular field and have been appointed by a public prosecutor or a judge to provide information or carry out an investigation. Under the Experts in Criminal Cases Act, the public prosecutor must inform the defence of any instructions given to an expert. At that point, the defendant may submit requests for further investigation or provide guidance on the investigation to be carried out.

Counter-investigation

Once the findings have been announced, the defence has two weeks to request a counter-examination, stating the grounds for doing so. The defence must also specify which – registered – expert should carry out the investigation, which must be equivalent to the initial investigation. If the request is granted, the expert will be given access to the investigation material and the relevant data from the initial investigation. This may, for example, involve DNA analysis, toxicology or a financial analysis.

The safeguards set out in Articles 150a to 150c of the Code of Criminal Procedure do not apply to specialist police investigations recorded in an official report. However, if the evidence relies predominantly on such an investigation, a prompt, reasonable and well-reasoned request by the defence for a counter-investigation is unlikely to be rejected out of hand. An alert defence is of the utmost importance.

If the public prosecutor refuses to authorise a counter-investigation

The public prosecutor may only reject a request from the defence if he considers it not to be in the defence’s interests. If the defence requests an expert who is not registered in the Dutch Register of Judicial Experts (NRGD), this request must be addressed to the examining magistrate.

If the public prosecutor refuses to carry out a particular investigation, he must inform the defence of this, giving reasons. From that point onwards, the defence has two weeks to ask the investigating judge to appoint an expert after all, or to extend the investigation. The previous request to the public prosecutor may then be repeated.

Access to the underlying research material

Under the law, the expert who is to carry out the counter-investigation is entitled to access the underlying investigation material. Access does not mean that they are permitted to take the material away. It may also be the case that the material has been lost or is no longer available. Even if deliberate destruction by or on behalf of the Public Prosecution Service can be ruled out, this does not mean that the initial investigation report is no longer usable. That depends on the individual case. The defence would, however, be well advised to request that the material be preserved at an early stage. Under the proposed new Code of Criminal Procedure, it is suggested that the counter-expert should only be granted access to the investigation material and not to the initial report; however, this is not yet law.

Commissioning your own research

The defence may commission its own expert investigation. This has both advantages and disadvantages. The advantage is that a particular investigation need only be submitted in the proceedings if it actually strengthens the defence’s case. The disadvantage is that the costs are borne by the defence and it is not certain in advance what weight the court will attach to this expert report. It is also important to note that an expert who has not been appointed but has been engaged by the defence does not automatically have the right of access to the investigation material. If access to this material is essential, one must consider whether commissioning an investigation independently is still worthwhile. If the report produced independently is ultimately shown to contribute to the court’s decision-making, the costs of the expert investigation may, upon request, be reimbursed to the defendant.

Applications made at the hearing

The later requests for investigations are made, the more difficult it becomes to have them granted. The case will then have to be adjourned, and it will take more time and effort to carry out certain supplementary or counter-investigations. Requests will then only be carried out if the court deems this necessary to answer the key questions relevant to the judgement. That bar is set high. There is a strong likelihood that an existing report can then only be challenged and that the scope of the enquiry can no longer be directed.

An effective defence therefore begins at an early stage. Not just at the hearing.

Mr. D.M. Penn

Sources: Articles 150a, 150b and 315 of the Code of Criminal Procedure (wetten.overheid.nl, version 1 July 2026); Public Prosecution Service guidelines on technical investigations and expert examinations; R.A. Hoving, ‘The Expert’, Handbook of Criminal Cases, Chapter 25 (up to and including 27 December 2023). T&C Criminal Procedure, commentary on Articles 150, 150c and 343 of the Code of Criminal Procedure (M.J. Dubelaar, Wolters Kluwer, current as at 5 August 2026). Parliamentary Papers II 2006/07, 31116, No. 3 (Explanatory Memorandum to the Act on Experts in Criminal Cases); reader on expert evidence in criminal proceedings (Recht in de Regio, 2021).