In this work we consider the mobility of personal on-line identifiers. People change the identifiers through which they are reachable on-line as they change jobs or residences or Internet service providers. This kind of personal mobility makes reaching people on-line error-prone. As people move, they do not always know who or what has cached their now obsolete identifiers so as to inform them of the move. Use of these old identifiers can cause delivery failure of important messages, or worse, may cause delivery of messages to unintended recipients. For example, a sensitive email message sent to my now obsolete work address at a former place of employment may reach my unfriendly former boss instead of me. In this paper we describe HINTS, a historic name-trail service. This service provides a persistent way to name willing participants on-line using today?s transient on-line identifiers. HINTS accomplishes this by connecting together the names a person uses along with the times during which those names were valid for that person. A correspondent who wishes to reach a mobile person can use an obsolete on-line name for that person, qualified with a time at which the on-line name was successfully used; HINTS resolves this historic name to a current valid on-line identifier for the intended recipient, if that recipient has chosen to leave a name trail in HINTS.