What evidence is presented in a criminal trial?
In criminal proceedings, evidence is everything. Without sufficient evidence, there can be no conviction, no matter how convinced the judge may be that someone has committed a crime. This statement is not rhetorical: it is the core of the system of guarantees that protects citizens against the punitive power of the State. The principle of the presumption of innocence requires that it is the prosecution that proves the defendant’s guilt beyond any reasonable doubt, and that proof can only be achieved through evidence that has been lawfully obtained and presented with all procedural guarantees.
But evidence does not operate on its own. It is not enough for evidence to exist: it must be admitted by the court, properly presented during the trial, and assessed in a rational and reasoned manner in the judgment. Each of these steps has its own legal logic, and understanding them is essential both for the defence —which needs to know what evidence can undermine its position and how to challenge it— and for the prosecution —which must build a solid evidentiary case capable of withstanding scrutiny by the defence.
In this article, we explain what types of evidence are presented in a criminal trial, how they are proposed and admitted, their characteristics and limitations, and what happens when evidence has been obtained improperly. A clear understanding of criminal evidence is essential to understand why some trials end in acquittal despite the apparent strength of the prosecution, and why others result in conviction even when the accused denies the facts.
What is evidence in criminal proceedings?
In legal terms, criminal evidence is any lawful means through which the parties attempt to demonstrate to the court the truthfulness of the facts they assert. Its purpose is to provide the judge or court with the necessary elements of conviction to decide, with the greatest possible certainty, whether the accused committed the acts attributed to them and, if so, under what circumstances.
For evidence to be assessed by the court, several essential conditions must be met. First, it must have been proposed at the appropriate procedural stage —usually in the prosecution or defence submissions— and admitted by the court before the trial begins. Second, it must be presented during the trial, with full guarantees of publicity, adversarial process, and immediacy. Third, it must have been obtained lawfully, respecting the fundamental rights of the suspect. If any of these conditions is not met, the evidence may be declared null and inadmissible.
The assessment of evidence corresponds to the court with full discretion, although subject to the limits of logic, experience, and scientific knowledge. The court is not obliged to believe anyone or accept any expert report simply because it has been submitted by the prosecution or the defence: it must provide reasoned justification as to why it gives credibility to certain evidence and not to others, and that reasoning allows the parties to understand the judgment and, where appropriate, to appeal it.
Witness evidence: the testimony of those who know
Witness evidence is one of the most commonly used forms in criminal proceedings and consists of the testimony of individuals who have direct or indirect knowledge of the facts being tried. Witnesses are called to testify about what they saw, heard, or know in relation to the offence and are examined both by the party that calls them and by the opposing party.
The evidentiary value of witness testimony depends largely on the credibility of the witness, and this is where the lawyer’s work becomes especially important. During cross-examination, the defence may challenge the witness’s credibility by highlighting inconsistencies between their testimony at trial and what they stated during the investigation, pointing out possible motives to lie, or demonstrating that the witness could not have seen or heard what they claim for physical or logical reasons.
There is a special category of witness that deserves particular mention: the victim testifying as a witness. In many offences —assaults, robberies, fraud— the only direct evidence available is the victim’s testimony. Courts have accepted that the victim’s statement may be sufficient to support a conviction when certain conditions are met: absence of improper motives, internal coherence of the account, consistency in the accusation, and peripheral corroboration through other evidence. However, it is also true that contradictory, implausible, or unsupported testimony may not be sufficient to overcome the presumption of innocence.
Hearsay witnesses —those who did not directly witness the events but know about them through what someone else told them— have more limited evidentiary value. Their testimony may support other evidence but is unlikely to serve as the primary basis for a conviction.
Expert evidence: when specialists are needed
There are facts whose understanding or verification requires specialised knowledge beyond the judge’s general knowledge. For these cases, expert evidence exists, consisting of a report prepared by an expert in a specific field: forensic medicine, accounting, IT, engineering, psychology, handwriting analysis, toxicology, ballistics, and many others.
In criminal law, some expert reports are absolutely decisive for the outcome of the case. A forensic report confirming or ruling out injuries, an accounting report quantifying tax fraud or embezzlement, a toxicology analysis determining the composition of a substance, or a DNA report linking or excluding the accused from the crime scene are examples of expert evidence that can be decisive.
Expert evidence carries a risk that should be understood: the tendency of courts to give excessive credibility to technical reports due to their apparent scientific objectivity. However, experts are not neutral: they are appointed and paid by the parties, and their conclusions may be influenced —consciously or unconsciously— by who has hired them. The defence always has the right to present its own expert to challenge the prosecution’s report, and a well-chosen defence expert can completely change the court’s assessment of key technical aspects.
During the trial, experts appear to confirm their reports and answer questions. Cross-examination of the opposing expert is a powerful tool: it may reveal methodological flaws, unsupported conclusions, or lack of expertise.
Documentary evidence: what is recorded in writing
Documentary evidence includes all documents submitted to prove facts: contracts, invoices, bank statements, emails, text messages, photographs, videos, medical reports, certificates, accounting records, and any written or audiovisual material relevant to the case.
In modern criminal proceedings, digital documentary evidence has become increasingly important and presents new legal challenges. WhatsApp messages, emails, social media posts, and call records are often decisive, but must meet strict requirements to be valid. A screenshot without proof of authenticity, or messages obtained by accessing a phone without consent or judicial authorisation, may be declared null.
Documentary evidence is introduced at trial through submission and examination. Parties may request full or partial reading and argue about its meaning. The defence may challenge authenticity and present its own documents.
Line-ups and identification evidence (investigation phase)
When identifying the perpetrator is key, specific mechanisms exist, such as identification line-ups. For validity, strict guarantees must be followed. Improper procedures may be challenged.
Other identification evidence includes DNA, fingerprints, video analysis, and handwriting analysis.
Site inspection and reconstruction (investigation phase)
A site inspection involves the judge examining the location of the events. Reconstruction may be used to verify versions of events.
Illegally obtained evidence: when form matters as much as substance
A key guarantee is the prohibition of evidence obtained in violation of fundamental rights. The “fruit of the poisonous tree” doctrine excludes both illegal evidence and derived evidence.
Common violations include unlawful searches, intercepted communications, coercion, and illegal access to devices.
Challenging unlawful evidence is a key defence task. If key evidence is excluded, the case may collapse.
Who must prove what? Burden of proof
The burden of proof lies entirely with the prosecution. The accused is presumed innocent.
If reasonable doubt remains, the court must acquit under the principle “in dubio pro reo.”
The defence may still present evidence to create doubt.
Frequently asked questions
Can new evidence be proposed during trial? Exceptionally, yes.
Can the court reject evidence? Yes, if irrelevant or unnecessary.
Do police statements have evidentiary value? Not by themselves.
Can WhatsApp messages be used? Yes, if authentic and lawfully obtained.
Is victim testimony enough? Yes, under strict conditions.
