If the police charge you with a crime, they need evidence to back up their claims. That could be verbal evidence given by an eyewitness, or physical evidence, such as a gun used to shoot someone.
Finding a way to challenge the validity of that evidence may be crucial to your defense. In the case of physical evidence, there are three principal ways to do so.
You can challenge when the police found the items
The police accuse you of stabbing someone in July 2020. In July 2021, they found a knife at the crime scene and claim it was the weapon used in the attack. You argue that if it were, they would have found it at the time. Police can only seal off a crime scene for so long. After that, there are many ways items could end up at the site.
You may challenge how the police acquired the evidence
The police do not have the right to go around looking for evidence everywhere. Officers can only gather evidence from a personal property such as your home or car if they have proper authorization. Typically this means they need a search warrant, although there are exceptions. For example, if they stop you and see evidence on view in your car, they can take it.
You can challenge how the police stored the items
A police officer takes a bag of white powder from your house and takes it back to the station, despite your protests that it is monosodium glutamate. They need clear records to show the bag of powder the lab confirmed as cocaine was the same one they collected from you.
There are many ways to contest criminal charges. Understanding them will be essential to a successful defense.