Live-cell and live-tissue imaging using fluorescence optical microscopes presents an inherent trade-off between image quality and photodamage. Spatially-controlled illumination microscopy (SCIM) aims to strike the right balance between obtaining good image quality and minimizing the risk of photodamage. In traditional imaging, illumination is performed with a spatially-uniform light dose resulting in spatially-variable detected signals. SCIM adopts an alternative imaging approach where illumination is performed with a spatially-variable light dose resulting in spatially-uniform detected signals. The actual image information of the biological specimen in SCIM is predominantly encoded in the illumination profile. SCIM uses real-time spatial control of illumination in the imaging of fluorescent biological specimens. This alternative imaging paradigm reduces the overall illumination light dose during imaging, which facilitates prolonged imaging of live biological specimens by minimizing photodamage without compromising image quality. Additionally, the dynamic range of a SCIM image is no longer limited by the dynamic range of the detector (or camera), since it employs a uniform detection strategy. The large dynamic range of SCIM is predominantly determined by the illumination profile, and is advantageous for imaging both live and fixed biological specimens. In the present review, the concept and working mechanisms of SCIM are discussed, together with its application in various types of optical microscopes.