In covert participant observation, the researcher observes the subjects without their knowledge or consent. This is in contrast to overt participant observation, in which the researcher's presence and role are known to the subjects.
Covert participant observation is controversial because it raises ethical questions about privacy, consent, and deception. Some researchers argue that covert participant observation is necessary to obtain data that would not be available through other methods, while others argue that it is an invasion of privacy and can be harmful to the subjects.