Mindfulness describes a state of mind characterized by paying attention to the present moment while maintaining a non-judgmental and accepting attitude. Cognitive models postulate that practicing mindfulness improves attentional and executive control and such beneficial effects are partially confirmed by empirical studies. However, the dose-response relation between mindfulness and improvements in cognitive control is less understood. It has been argued that initial phases of the practice produce unspecific outcomes, which may also be attainable by induced relaxation. Therefore, this dissertation examined the effects of short mindfulness trainings of varying duration and frequency on attentional control (alerting, orienting and executive attention; Study 1) and executive functions (updating, inhibition and task switching; Study 1 & 2) with separate reaction-time tasks. In Study 3, it was investigated if improvements in cognitive control following short mindfulness trainings transfer to more complex behavioral control, namely the suppression of stereotype-biased behavior. Across three studies, mindfulness (breathing meditation) was contrasted with relaxation (progressive muscle relaxation) to investigate whether short mindfulness trainings produce specific effects or whether an initial underlying mechanism is state relaxation. Podcast listening was utilized as a passive control condition to control for effects of repeated testing. Results of Study 1 revealed similar improvements for updating and executive attention following a mindfulness induction and relaxation compared to the passive control condition. Results for inhibition and task switching suggested differential, albeit not superior, effects following mindfulness compared to relaxation. No improvements were present for the alerting and orienting network. Thus, results suggested partly similar, partly differential mechanisms of mindfulness induction and relaxation. In Study 2, no effects on the executive functions updating, inhibition and task switching beyond repeated testing were found following an induction (Exp 1) and a brief training (Exp 2) in mindfulness and relaxation. Based on these findings, it could not be concluded that effects in the initial stages of mindfulness training differ from those of relaxation. It was discussed that effects on executive functioning following short mindfulness trainings might be too transient to be reliably measured in pre-post experimental designs. In Study 3, compared to a passive control condition, a mindfulness induction (Exp 1) and a brief training (Exp 2) increased the effect of stereotype bias on decision-making while relaxation reduced the effect. These findings provided additional evidence for differentiable effects of short mindfulness practices compared to relaxation. However, findings for mindfulness were not in line with theoretical propositions and previous research, and improvements following relaxation may suggest that processes outside of cognitive control affected performance. In summary, the results of the three studies suggest partially differential, partially overlapping mechanisms of short mindfulness trainings and relaxation. The present dissertation contributes to understanding the development and specificity of cognitive effects in the initial phases of mindfulness practice and discusses implications for research in the field.