Actions aim to produce effects in the environment. To accomplish this properly, we not only have to recruit the appropriate motor patterns, but also we must be able to monitor whether an intended effect has ultimately been realized. Here, we investigated the impact of such effect monitoring on performance in multitasking situations: Multitasking basically means to produce and monitor multiple actions and effects in fast succession. We show that effect monitoring cannot run in parallel, without causing processing decrements, with a second task. Also, monitoring of effects that are spatially incompatible to a response seems to take longer than the monitoring of spatially compatible action effects (Experiments 1 through 4). We further argue that effect monitoring is essential toward learning of response-effect associations, as it captures not only anticipated action effects, but also unpredictable occurrences in the environment (Experiments 5a and 5b).