I have to have a basic understanding of chemistry, physics, anatomy, and math. I am always identifying drugs, taking samples, and making calculations. You need to know which evidence needs to be collected quickly because it may deteriorate. You need to understand the threat of blood-born pathogens. And, you have to have a strong stomach.
Homicides can really be nasty scenes.
Beyond that I have stood in on autopsies in order to make sure that specific pieces of evidence were collected. You have to know what you need, what you are talking about, and what questions need to be asked. This is why part of my training has taken place in medical schools. This attention to details will not only help you solve the case at hand, but maybe even other cases. In one instance the collection and identification of one fingerprint in a crime scene, in turn led to the solving of 40 other cases.