The mammalian cortex is complex, consisting of many functionally distinct areas. Understanding how this diversification occurs remains a major goal. Interneurons in the adult mouse cortex are diverse, they can be defined based on developmental origin, distinct morphology, physiological and molecular properties (e.g. parvalbumin-expressing basket cells). Despite making up approximately 1/6 of the total neuronal population we do not currently have a cortex-wide map of interneurons.Using transgenic mice to label cells we have mapped the distribution of all interneurons, of 2 broad populations (MGE/CGE derived), and of 5 discreet populations (PV-Basket, T-shaped/fanning out martinotti, VIP-bipolar and CCK-basket) across 26 distinct regions in the adult mouse cortex. Interneuron distribution is heterogeneous across the cortex, we demonstrate this using: density, ratio to pyramidal cells, and total proportions. The final number of cells in the cortex is fine-tuned via programmed cell death; which occurs during early postnatal development and is important for maintaining the correct cellular E/I ratio. This process has not yet been examined across the entire cortex. We used transgenic mice in which GABA cell populations are ‘immortalised’ to prevent programmed cell death and to estimate the proportion of cells undergoing the process. The levels of death across the cortex ranges from 0-40%. Additionally, different cells are affected to varying degrees across regions. A heterogeneous distribution is present when all interneurons are immortalised suggesting that there is also differential deployment. Further investigation could establish whether these differences in inhibitory populations contribute to the functional specialisation of cortical areas.