Methods and apparatus are disclosed to monitory online activity. An example apparatus includes at least one memory, instructions, and processor circuitry to execute the instructions to generate a first database to store first uniform resource locators collected from first client devices, the first client devices associated with panelists, generate a second database to store second uniform resource locators collected from second client devices, one or more of the second client devices associated with one or more unidentified users, associate at least one of the second uniform resource locators as online activity of a first panelist of the panelists based on at least a portion of one of the first uniform resource locators matching at least a portion of the one of the second uniform resource locators, and store an association of the online activity and an identifier, the identifier to identify the first panelist.