Program slicing can be used as a support for program comprehension, because it allows a large program to be divided up into smaller slices, each of which can be understood in isolation from the rest. As such, slicing facilitates the familiar approach of 'divide and conquer'. Union slicing (the union of dynamic slices) is a useful technique for approximating a precise static slice. For program comprehension (and many other applications) it is often important that the union slice be an executable program, rather than merely a collection of statements which are relevant to the slicing criterion. This paper presents an algorithm for computing executable union slices, using conditioned slicing. A case study is used to illustrate the algorithm and how the executable union slice is preferable to the (possibly non executable) union slice. The paper also shows, briefly, that the approach has wider applications than comprehension.