Field work is work done outside of the Crime Labs.
Both Crime Scene Investigators and Detectives do field work, while lab techs do most of the lab work. CSIs sometimes also do lab work, especially if the work is pressing or there's a shortage of lab techs.
In Miami and New York, whenever the CSIs go out to process a scene, they wear normal clothing but they also carry guns for protection.
In Las Vegas, though, all the CSIS including some lab techs are given stab vests whenever they go out into the field to presumbly protect them from any possible bullets.
These vests also identify them as CSIs and give their surnames as well, for example, Greg Sanders wears one and his surname, Sanders is on the label.
- Field work is inherently more risky and dangerous than lab work due to the risk of both CSIs or cops getting attacked, shot or killed by possible suspects or criminals. This has been seen happening a few times in both Las Vegas and Miami with the suspects killing cops or innocent bystanders who were in the wrong place at the wrong time.