This chapter introduces an interactive debugging methodology based on the Waveform Analysis Language (WAL). It utilizes virtual signals, which are signals that are injected into the waveform after simulation at analysis time. These virtual signals are backed by WAL code that is evaluated when a virtual signal is read. By injecting these signals into a trace, existing signals can be replaced. This allows a new debugging methodology in which only small parts of a design are reevaluated after a bug fix, thus removing the need for expensive simulation runs.