Emotion regulation is a broad term referring to intensifying, maintaining or reducing either positive or negative emotions. Cognitive reappraisal and expressive suppression are two forms of emotion regulation that have been most extensively studied, especially in the context of regulating negative emotions. Cognitive reappraisal is defined as altering a way one perceives an emotional situation in order to change its emotional salience. Expressive suppression refers to blocking facial expressions of emotion. Despite a large body of research, it is not clear whether these strategies have cognitive consequences. Therefore, the aim of this thesis was to understand whether cognitive reappraisal and expressive suppression are cognitively demanding. To address this question, we conducted five experiments. In every experiment participants were presented with unpleasant and neutral pictures and asked to either view the pictures naturally or to regulate their emotions according to provided instructions. Emotion regulation success was measured by self-reported ratings of negative emotions experienced and experienced arousal. Additionally, in Chapters 3 and 4 emotion regulation success was measured by electromyography (EMG), a technique that allowed us to also assess temporal dynamics of emotion regulation. In two experiments (Chapter 3) we employed a divided attention paradigm to investigate whether performing a regulation task would impair performance on a concurrent task. The results of these experiments motivated two electroencephalography (EEG) studies focusing on the cognitive reappraisal strategy. In Chapter 4 EEG provided a continuous measure of cognitive reappraisal consequences and allowed for further exploration of its temporal dynamics. In Chapter 5 EEG was used to investigate whether cognitive load associated with reappraisal would impair a concurrent involuntary process of novelty detection in an auditory oddball paradigm. Finally, in Chapter 6 we investigated whether cognitive reappraisal influences emotional memory and whether this potential influence could be explained either through stimulus elaboration or changes in arousal. Our findings suggest that cognitive reappraisal is an effective emotion regulation strategy, evident in behavioural and EMG measures. We found that cognitive reappraisal up- and down-regulation impaired performance on a concurrent cognitive task, suggesting that it might be cognitively costly. The claim that reappraisal down-regulation is cognitively demanding was further supported by the finding that it increased the amplitude of the late positive potential (LPP). However, cognitive reappraisal down-regulation did not impair involuntary novelty detection, indexed through the amplitudes of the novelty P300. Moreover, in Chapter 6 we did not observe an impact of cognitive reappraisal on emotional memory. These memory results did not support the claim that cognitive reappraisal influences emotional memory either via arousal or due to stimulus elaboration. EMG results as well as results of evoked amplitude and EEG oscillatory power suggested that cognitive reappraisal has a unique temporal dynamics, compared to the no-regulation control condition, and that changes due to reappraisal emerge over time. Replicating previous research, expressive suppression reduced facial expressions of emotions but had no influence on experienced emotions or arousal. We did not find support for the claim that expressive suppression was cognitively costly. To sum up, our results support the notion that cognitive reappraisal is an effective way to voluntary regulate negative emotions. However, it results in cognitive cost. Finally, it has its own temporal dynamics as shown by EEG and EMG results.