RRDtool (Round Robin Database tool) is a system to store and display time-series data (e.g. network bandwidth, machine-room temperature, server load average, or even the height of surfing waves on La Jolla Shores). It stores the data in a very compact way, aggregating at stepwise coarser granularity as it archives further back in time, so as to maintain manageable archive size, RRDtool presents useful graphs by processing the data to enforce a certain data density. RRDtool can be used either via simple wrapper scripts (from shell or Perl) or via user-friendly frontends that poll network devices. Tobi Oetiker developed part of RRDtool during his summer 1999 sabbatical with CAIDA. He continues to support it from his home institution, the Swiss Federal Institute of Technology.