The ability to estimate and count the number of objects in a collection is held to ground the development of symbolic numerical thinking. Nevertheless, we still lack understanding the neural code that supports the access to numerical quantity in these basic quantification tasks. In this thesis we present three experiments that, making use of functional imaging technique (functional Magnetic Resonance Imaging and Positron Emission Tomography) investigate the neural basis of estimation and counting in human adults. We first investigated the special status that small collections have in quantification. Small collections of up to 4 items are quantified fast and effortless, via a process called subitizing, in opposition to counting more than 3-4 items, where performance gets slow and error prone. It is held that this dichotomy reflects two qualitatively different processes. Using PET and fMRI, we showed that there is no brain circuit that is specialised for subitizing or counting: a bilateral occipito-fronto-parietal system is activated in both. However, the intensity of the activation in this circuit shows a sudden discontinuity between the two processes. We therefore demonstrated that subitizing and counting emerge from a qualitatively different recruitment of a set of brain regions, but that they are not anatomically segregated. In the third study we used fMRI to compare verbal counting to non-verbal numerosity estimation, using visual and auditory stimuli. Results showed that numerosity estimation emerges from the activity of a strongly right lateralised fronto-parietal circuit which is almost completely independent from the modality of stimuli presentation. This showed both the abstract and the non-linguistic nature of the numerosity-related computations. Counting also was independent of input modality, and recruited additional bilateral posterior parietal regions and linguistic regions of the left hemisphere. This showed that counting and estimation are hierarchically organised, and confirmed the linguistic and attentional nature of counting.