The authors present a miniature, integrated fluorescence microscope designed for high-speed, cellular-level imaging in active mice. The device, weighing only 1.9 grams, is made from mass-producible components, including a semiconductor light source and sensor. This allows for imaging across areas of ~0.5 mm² in mice, enabling the concurrent tracking of Ca²⁺ spiking in over 200 Purkinje neurons across nine cerebellar microzones. During mouse locomotion, individual microzones exhibited large-scale, synchronized Ca²⁺ spiking, which was not detectable by previous techniques. The integrated microscope offers advantages over traditional microscopes in terms of optical sensitivity, field of view, resolution, mechanical flexibility, cost, and portability. It can be used for brain imaging in behaving animals, portable diagnostics, and high-throughput screening. The authors demonstrate its capabilities by imaging cerebellar microcirculation and tracking Purkinje neuron Ca²⁺ dynamics, revealing unexpected precision in vessel regulation and synchronized neural activity during motor behavior. The microscope's potential applications extend to in vitro assays, such as cell counting and diagnostic testing, and its portability makes it suitable for field use.The authors present a miniature, integrated fluorescence microscope designed for high-speed, cellular-level imaging in active mice. The device, weighing only 1.9 grams, is made from mass-producible components, including a semiconductor light source and sensor. This allows for imaging across areas of ~0.5 mm² in mice, enabling the concurrent tracking of Ca²⁺ spiking in over 200 Purkinje neurons across nine cerebellar microzones. During mouse locomotion, individual microzones exhibited large-scale, synchronized Ca²⁺ spiking, which was not detectable by previous techniques. The integrated microscope offers advantages over traditional microscopes in terms of optical sensitivity, field of view, resolution, mechanical flexibility, cost, and portability. It can be used for brain imaging in behaving animals, portable diagnostics, and high-throughput screening. The authors demonstrate its capabilities by imaging cerebellar microcirculation and tracking Purkinje neuron Ca²⁺ dynamics, revealing unexpected precision in vessel regulation and synchronized neural activity during motor behavior. The microscope's potential applications extend to in vitro assays, such as cell counting and diagnostic testing, and its portability makes it suitable for field use.