Held, the concept of “working time”, within the meaning of article 2(1) of the working time Directive, covered the entirety of periods of stand-by time, including those according to a stand-by system, during which the constraints imposed on the workers, objectively and very significantly, affected their ability freely to manage the time during which their professional services were not required and to pursue their own interests. Conversely, in the absence of such constraints, only the time linked to the work actually carried out during that period constituted “working time”. In order to determine whether a period of stand-by time was “working time”, only the constraints that were imposed on the worker, whether by national law, by a collective agreement or by the employer, could be taken into consideration. By contrast, organisational difficulties that a period of stand-by time might entail for the worker, which resulted from natural factors or the free choice of that worker, were not relevant, for example where there was a substantial distance between the worker’s residence and the workplace. Further, it was for the national courts to carry out an assessment of all the circumstances in order to determine whether a period of stand-by time constituted “working time”, as that classification was not automatic in the absence of a requirement to remain at the workplace. In particular, account should be taken of the consequences for the worker’s ability freely to manage their time resulting from the brevity of the time period within which they had to return to the workplace if recalled from stand-by. However, the consequences of such a time limit had to be specifically assessed, taking into account not only the other constraints imposed on the worker, such as the obligation to have specific equipment with them when returning to the workplace, but also the facilities that were made available to them, such as the provision of a service vehicle that permitted use of traffic regulations privileges. The national courts had also to have regard to the average frequency of the activities that the worker was actually called upon to undertake during the stand-by periods. However, the significant distance between the worker’s home and the workplace was not relevant. In the circumstances of the present case, the worker had to be able to reach the town boundary of their workplace within a 20-minute response time, in uniform with the service vehicle made available by the employer, using traffic regulations privileges and rights of priority attached to that vehicle. The average frequency of call-outs whilst on stand-by was not high. Accordingly, a period of stand-by time according to such a system as that in issue, constituted, in its entirety, “working time” within the meaning of article 2(1) of the working time Directive, solely if the constraints imposed on that worker very significantly affected their ability to manage their free time during the stand-by period, which was for the referring court to ascertain. However, employers could not establish periods of stand-by time that were so long or so frequent that they constituted a risk to the safety or health of workers, irrespective of those periods being classified as “rest periods” within the meaning of article 2(2) of the Directive (judgment, paras 38–41, 44–46, 48–50, 54, 60, 61, operative part).