We describe here the PET (standing for path exploration tool) system, developed in Bell Labs. This new tool allows an interactive testing of sequential or concurrent programs, using techniques taken from deductive program verification. It automatically generates and displays a graphical representation of the flow graph, and links the visual representation to the code. Testing is done by selecting execution paths, or, in the case of concurrent programs, interleaved sequences of code. The Pet system calculates the exact condition to execute path being selected, in terms of the program variables. It also calculates (when possible) whether this condition is vacuous (never satisfied) or universal (always satisfied). The user can then edit the path and select variants of it by either extending it, truncating it, or switching the order of appearance of concurrent events. This testing approach is not limited to finite state systems, and hence can be used in cases where a completely automatic verification cannot be applied.