Diamond is host to a wide variety of colour centres that show attractive optical and spin properties. Fluorescent defects in nanodiamonds are candidates for single-photon sources in quantum cryptography, qubits in quantum computing, optical labels in biomedical imaging, and sensors in magnetometry. The scope of this chapter is to analyse a particular colour centre in diamond: the nitrogen-vacancy (NV) centre. Of the hundreds of colour centres that have been identified and characterised in diamond, the NV centre has turned out to be one of the most appealing for scientific purposes. We will present the techniques that have been developed to synthesise artificial diamonds and to enhance the incorporation of NV colour centres into them. The characteristics of the host diamonds can influence the behaviour of the colour centres. This explains why a considerable part of the research around NV centres in diamond at present is actually focused on the optimisation of material purity and on surface functionalisation. We will also give a wide overview of the technological applications that have been identified for NV centres in nanodiamonds, The attention will be focused on showing in detail the NV centre structure and behaviour and how these are affected by the surrounding environment. Much has been done to improve the understanding of the NV centre, but there are still many questions that need answers. For example, what role do surface and strain play in emission properties. We will present the experimental apparatus we have designed and built to characterise NV centres in diamond nanoparticles and to address some of these questions. Our system combines a confocal microscope with an atomic force microscope (AFM). It allows a comprehensive analysis of the spin and optical properties of colour centres (e.g., spectral analysis, lifetime, autocorrelation function, blinking, bleaching, etc.) and of the relevant host diamond nanocrystals (e.g., size, distribution, etc.). We will also present some theoretical•computational modelling coupled with experimental data.