Despite the wide literature concerning attentional resources and memory, there is still a debate concerning the cognitive processes involved during multiple tasks. The single attentional resource theory suggests that each individual has a general single pool of attentional resources that can be shared amongst the different concomitant tasks. Dual task demand would result in a decrement in performance as the resources required exceed those available. In contrast the multiple attentional resource theory claims that each individual can draw from different pools of attention and that resources are not sharable: combining two tasks does not necessarily result in a dramatic decrement in performance. From this second theory arises the co-ordination hypothesis which suggests that the central executive may co-ordinate and implement the use of different pools of attention when an individual has to perform two tasks at the same time. In this thesis some aspects and predictions of these theoretical hypotheses are examined. The dual task effect is investigated under the manipulation of cognitive effort (Experiments 1, 3 and 4), practice (Experiment 2) and the combination of different processes (Experiment 6). Moreover, it has been reported in the literature that patients affected by Alzheimer Disease show a dramatic cost of concurrence, but there is still a debate as to whether the performance impairment is due to an impoverishment of the single attentional resource or, alternatively, to an impairment in the co-ordination function. The findings of this thesis provide some support in favour of a co-ordination impairment (Experiments 1,2,3 and 4). Finally the age effect (testing two groups of healthy adults, young and elderly) has been considered to establish whether there is a qualitative or quantitative difference between healthy elderly and Alzheimer patients (Experiments 1,2,3 and 4).