 As said above, the representation of actions and formalisms/logics to reason with them are very central to AI and particularly the field of KR. One of the main problems that one encounters in the literature on reasoning about actions in AI, and much more so than in mainstream computer science, is the discovery of the so-called frame problem (McCarthy and Hayes 1969). Although this problem has been generalized by philosophers such as Dennett (1984) to a general problem of relevance and salience of properties pertaining to action, the heart of the problem is that in a ‘common-sense’ setting as one encounters in AI, it is virtually impossible to specify all the effects by the actions of concern, as well as, notably, all non-effects. For instance, given an action, think about what changes if the action is performed and what does not—generally the latter is much more difficult to produce than the former, leading to large, complex attempts to specify the non-effects. But there is of course also the problem of relevance: what aspects are relevant for the problem at hand; which properties do we need to take into consideration? In particular, this also pertains to the preconditions of an action that would guarantee the successful performance/execution of an action. Again, in a common-sense environment, these are formidable, and one can always think of another (pre)condition that should be incorporated. For instance, for successfully starting the motor of a car, there should be a charged battery, sufficient fuel, …, but also not too cold weather, or even sufficient power in your fingers to be able to turn the starting key, the presence of a motor in the car, … etc. etc. In AI one tries to find a solution for the frame problem, having to do with the smallest possible specification. Although this problem gave rise to so-called defeasible or non-monotonic solutions such as defaults (‘normally a car has a motor’), which in itself gave rise to a whole new a realm within AI called nonmonotonic or commonsense reasoning, this is beyond the scope of this entry (we refer the interested reader to the article by Thomason (2003) in this encyclopedia). We focus here on a solution that does not appeal to nonmonotonicity (directly).