A description of EXDAMS, an online system designed to facilitate experimentation with new debugging and monitoring aids for higher-level computer languages. Instead of the usual manipulation of the source program while it runs, the EXDAMS system constructs a model of the program structure and a history tape of its operation containing all the control flow and variable-alternation information, after which no further interaction with the source program is required. The model serves to associate each value in the history with the source statement that produced it. Together with the model and symbol table, the history tape is run and observed by the user at a CRT terminal. The user controls the execution speed and can run the program history forward or backward (for flowback analysis). He can stop at any moment and switch from one EXDAMS debugging/monitoring aid to another. EXDAMS was designed for flexibility and experimentation rather than efficiency; however, use of the model to interpret the history, rather than having the history be self interpretable, saves about half the input/output.