Classical applications of superconducting quantum interference devices (SQUIDs), like biomagnetic measurements or precision voltage and current measurements have in almost all cases been carried out in a magnetically and electrically well shielded environment. Recently, the demonstration of highly sensitive HTS SQUIDs has increased the interest in SQUIDs for a broader range of applications like geophysical measurements or nondestructive materials evaluation. The operation of SQUIDs in an unshielded environment, however, leads in many cases to a significant reduction in their sensitivity due to numerous sources of interference and excess noise. The paper reviews recent work on aspects related to the operation of SQUIDs in unshielded environments.