The chapter covers subwavelength-localized optical fields and their interaction with matter. Localized fields contain evanescent waves, which decay exponentially away from their source region. To study the interaction of localized fields with matter, we introduce field-confining structures known as optical probes. To interact effectively with the sample, these optical probes are placed within the range of the evanescent waves and raster-scanned across the sample, a technique known as near-field optical microscopy. Given that optical probes inevitably interact with the sample, we start out with a series expansion of these probe–sample interactions, gaining insights into their nature and strength. We then discuss fundamental aspects of light confinement concepts and the corresponding optical probes, such as subwavelength apertures and resonant scatterers. This includes an exploration of how different probe designs influence the probe performance. Finally, we address probe–sample distance control and categorize various realizations of near-field optical microscopes according to the leading terms of the interaction series. This categorization helps to differentiate between different types of microscopes and their specific applications, providing a comprehensive overview of the field.