Described is technology allowing computer programs to participate in the copy and paste process at various times. A source program, from which data is requested to be copied to the clipboard, may participate as desired, including, for example, to cancel the copy, or to determine which data formats will be placed on the clipboard, including one or more custom data formats. A target program, to which data may be copied from the clipboard, may also participate as desired, including, for example, to cancel the paste, or to determine which data format will be pasted, which may be a custom data format. In one implementation, events provide notifications and APIs may be called to facilitate the communication.