Abstract In agent-oriented programming and planning, agents’ actions are typically speci?ed in terms of postconditions, and the model of execution assumes that the environment carries the actions out exactly as speci?ed. That is, it is assumed that the state of the environment after an action has been executed will satisfy its postcondition. In reality, however, such environments are rare: the actual execution of an action may fail, and the envisaged outcome is not met. We provide a conceptual framework for reasoning about success and failure of agents’ behaviours. In particular, we propose a measure that re?ects how ”good” an environment is with respect to agent’s capabilities and a given goal it might pursue. We also discuss which types of goals are worth pursuing, depending on the type of environment the agent is acting in.