A central feature of your right to a fair trial in a criminal case is the opportunity to ask the judge to exclude evidence. If a criminal defense lawyer believes something's wrong with a piece of evidence, they can file a motion to suppress. This is a formal request that the judge bars the prosecution from introducing the evidence at trial.

Unsurprisingly, the court will need to know why the motion is the right remedy before the judge will even consider it. A criminal defense attorney will usually push one of these four arguments.

Civil Rights Violations  

The rules of evidence prohibit cops and prosecutors from introducing any obtained evidence in violation of a defendant's civil rights. For example, the police can't just randomly search someone's house or vehicle for no reason. If the cops didn't have probable cause or a warrant for a search, then a criminal defense lawyer will race to suppress any evidence the state obtained.

Broken Chain of Custody

Whenever the government introduces evidence at trial, it has to prove that it handled the evidence without exposing it to the risk of manipulation. From the moment a cop picks something up at a crime scene, they have to document where and how they found it. Subsequently, they must accurately inventory the evidence without contamination. If they send the evidence to a lab for analysis, the cops also have to document every place it went, who handled it, and how. Even a single break in the chain of custody is enough to justify excluding the evidence from the trial.

Irrelevance

The government doesn't have the right to just carpet bomb the defense with irrelevant evidence. Similarly, they can't throw random evidence at a jury or judge in the hope that something might click. A prosecutor has to explain in detail what the evidence is and how it relates to the case. Otherwise, a criminal defense attorney will jump at the chance to exclude the evidence.

Scientific Invalidity

Forensic evidence often enters trial through scientific analysis. However, there is a lot of junk science in the world. Worse, many prosecutors are happy to throw junk science at jurors to make their cases sound deep and irrefutable. A criminal defense lawyer, though, has the right to question the scientific basis of the state's claims about the alleged evidence. If the supposedly scientific methods can't withstand scrutiny, the defense will ask to exclude it. Even if the judge allows the evidence, the defense may bring in experts to refute the evidence's validity.