Based on the principles of the brain-computer interface (BCI) technology, Neurofeedback (NF) training is conceived as a self-regulation technique. Its standard paradigm is the operant conditioning with the aim of teaching individuals to control their own brain activity (Frank et al., 2010). Specifically, the information from the brain waves is represented in real time as an image or audio and individuals are rewarded when a value, usually the wave amplitude, changes according to the selected thresholds. These thresholds will be established according to the marked goals in relation to the pattern of the electroencephalogram (EEG), whether it is dysfunctional or not. The ultimate goal consists in receiving direct information about the changes that occur in the physiological signal, that is, a feedback, and thus learn to modify it (Carrobles and Godoy, 1987). Therefore, the brain can improve both its functionality and structure by observing and listening to real-time multimedia representations of the brain activity. On another note, recent literature suggests that cortical activity modulates and relates to cardiovascular and endocrine functions. Moreover, some experts have considered both functions as possible indicators of greater or less inhibition exerted by the frontal cortex, a factor to be taken into account in some symptomatology. Up to the present time, the NF technique has been well studied for the treatment of various disorders (epilepsy, anxiety, depression, attention deficit hyperactivity disorder (ADHD), etc.). Likewise, the clinical population has not been the only beneficiary of this technique, as in the last decades the performance improvement in non-pathological population has also been an aim of study. However, recent statistics report that a third of the population does not get any benefit when receiving NF training. Therefore, there is an imperative need to find out what personality factors make a person more receptive or respondent to this technique. Among the main factors suggested by recent literature (Linden, 2014) to be evaluated are: locus of control, repeated evaluations of thoughts and strategies implemented, the two motivation systems (approach or avoidance), and the perception of or the sensitivity to rewards. Also, in order to unravel the factors that may be influencing the results of this technique, a need arose to optimize the designs that had been applied so far in NF studies (Thibault et al., 2016). Briefly, the most widely used design during the last decades included an experimental condition, which received a real NF training, and a sham condition, receiving a simulated and non-contingent training from a previously recorded session. While this latter condition is necessary to suggest the real therapeutic effect of NF, this design is not robust enough to ensure the impact of this technique as a placebo effect (Thibault et al., 2016). Thus, the optimization proposed in recent research includes a control condition in which no intervention or feedback is administered. This control condition, therefore, is considered necessary in order to control and determine the importance of exposure of participants to NF training consciously. Hence, the studies that have formed this thesis consists: on one hand, cross-sectional studies (studies 1 and 2) pursue, based on an optimized experimental design, the analysis of influential personality factors and the immediate psychophysiological effects obtained after a single session of Neurofeedback training; and, on the other hand, a longitudinal study of ten NF training sessions in a case of anxious symptomatology. Additionally, the studies of this thesis have included the impact that NF training can have on heart rate (HR) and salivary cortisol as aims to be addressed. The aim of the study 1 was to implement a double-blind well- controlled design to evidence differences between an actual effect of NF training and the existence of a placebo effect. Forty healthy volunteers were randomly assigned to one of four conditions: a) theta (down-training theta amplitude), b) SMR/theta (up-training SMR and down-training theta amplitudes simultaneously), c) sham (fake NF training), and, d) control. Attentional function (Stroop test), mood (PANAS) and anxiety (STAI) were measured in a pre/post design. Heart Rate (HR) was recorded transversally to evidence how NF influenced the Autonomic Nervous System (ANS). Results showed that successful modifications of electroencephalogram (EEG) amplitudes were achieved in the theta and SMR/theta conditions, whereas sham condition experienced a contrary effect and the control condition remained unchanged. Sham participants obtained significant impaired attentional performance, as well as an increase in situational anxiety and negative mood. Also, NF training impacted significantly different in terms of ANS activation: theta participants obtained a decrease in activation whereas SMR/theta and sham participants experienced the contrary. Meanwhile, the aim of the study 2 was to show that personality factors are significantly related to NF trainability and changes in brain waves’ amplitudes accomplished by participants. Forty healthy volunteers were randomly assigned to one of three conditions: experimental (SMR/theta), sham (fake NF training) and control. Locus of control (Rotter test), re-appraisal process performed by individuals (TCQ) and the two motivation systems (BIS/BAS) were assessed before volunteers underwent NF training (real or sham) or control task. Once NF training was complete, experimental participants were categorized as either NF respondents or non-respondents. Heart Rate (HR) was recorded transversally to show how NF influenced the Autonomic Nervous System (ANS). Only the experimental condition and, specifically, NF respondents, displayed significant changes in EEG rhythms. Even more, only respondent participants showed significant relations between physiological changes (EEG and HR) and personality factors. Locus of control and performing re-appraisal processes were significant predictors of the ability to gain control over brain activity and changes in HR throughout NF training. Finally, the principal aim of the longitudinal study (n = 1) was to restore an optimal inhibitory control over anxious symptomatology and to analyse immediate effects on anxiety after a ten-session NF training in a subclinical case. A participant suffering from an anxious symptomatology underwent 10 sessions of Neurofeedback, in a protocol consisting of uptraining the beta1 rhythm (16-21 Hz) while downtraining the theta (4-8 Hz) band. State anxiety and salivary cortisol levels were measured during each of the 10 sessions following a pre/post design. Initial and final examinations of anxiety symptoms (STAI) and sustained attention (Toulouse test) performance were also implemented. The final evaluation revealed that levels of anxiety fell within a normative range and that sustained attention had improved. A t-test for related samples disclosed a significant improvement of beta1 amplitude across the sessions, without modifications in untrained bands. Also, a significant inverse correlation between beta1 amplitude and salivary cortisol was detected, suggesting that brain activity could be considered a marker of anxiety. The main conclusions from both cross-sectional and longitudinal studies are an effectiveness of the Neurofeedback training technique with immediate, as well as short-medium term, physiological and psychological effects. Likewise, personality factors analyses have clarified that an internal locus of control and a repeated evaluation of thoughts and strategies implemented could predispose to show greater training capacity, what would explain why some people are more respondent than others to Neurofeedback or other feedback-based techniques. Finally, results suggest that Neurofeedback training impacts differently on ANS depending on the training protocol used.