An assessor is a person who is appointed to a given case when it is necessary to determine circumstances requiring specific information. This is to do with specialist technical, scientific, medical or artistic knowledge, which is indispensable for better understanding of a given circumstance or evidence concerning a case. For instance, it would be necessary to get the opinion of a doctor to determine and explain what sort of damage to health had been inflicted to a victim of a crime and how the wounds were inflicted.
In some cases, it is necessary to appoint a psychologist or a psychiatrist who would provide a delineation of the defendant and evaluate their mental state. The same thing happens when an computer specialist assessor is appointed, who would determine how a given software or a computer were used to commit a crime. In such cases, the doctor, psychologist, computer specialist etc. use their specialist knowledge to explain how a crime was committed.
Appointment of an assessor may take place at the request of a litigant (e.g. a victim) or the court or prosecutor.
Depending on the court's decision, an assessor would deliver an opinion verbally or in written form.

The opinion should include:

  1. the name, surname, degree, speciality and job title of the assessor,
  2. names and surnames and other data of the persons involved in forming the expert opinion, with assignation of activities performed by each of them,
  3. in the case of an institutional opinion – the full name and address of the institution are required,
  4. the time of the conducted research and the opinion issuing date,
  5. a report on the performed activities and observations, as well as the conclusions based on them,
  6. signatures of all the assessors who had taken part in issuing the expert opinion.

An assessor's expert opinion is a judgement expressed by an independent person with appropriate specialist knowledge, who has no agenda connected with the final settlement of the case. Such an opinion is to make evaluation of a situation or evidence easier for the court. An assessor receives appropriate remuneration for their work.