The surface of the kidney presents a unique opportunity to study fundamental processes in the living animal. Here blood vessels and kidney tubules are accessible to direct microscopic observation. This situation has been used to elucidate kidney functions under normal and pathological conditions. A number of observations on function in the intact kidney can be made through the microscope. We can observe the flow of blood in the renal microcirculation on the cortical surface. We can identify proximal and distal tubules, and measure their widths. We can follow the flow of tubular fluid with the use of dyes. We can watch fluid reabsorption in tubular segments isolated by oil droplets. With photography, these observations can be recorded and quantitated. In this monograph, we will survey the methods and results of in vivo microscopy of the kidney cortex. We will consider the advantages and limitations of these methods, and discuss their contributions to our understanding of kidney physiology. A general discussion of kidney function may be found in Ref. [127]. The reader is also referred to several recent reviews and books on the kidney [28, 41, 43, 66, 72a, 84, 95, 120, 128, 136, 140, 194, 203, 204, 210, 216].