There are three varieties of surveillance: (1) no electronic devices, (2) electronic surveillance of communications, and (3) use of devices that do not intercept communications. Warrantless use of personal observations is acceptable if this does not invade privacy. Homes are given special protection. Electronic surveillance requires prior consent or authorization. Devices that collect information other than communications, such as pen registers, tracking devices, or heat-seeking devices, have fewer restrictions, but may require a warrant. Evidence that may favor a suspect, or that may be irreplaceable, should not be destroyed.