Fiber optofluidic lasers (FOFLs) integrate optical fiber microcavities and microfluidic channels to achieve ultrasensitive biochemical sensing, which is one of the frontiers in the development of “Lab-on-fiber”. FOFLs inherit the advantages of high sensitivity, high signal-to-noise ratio, and narrow linewidth from lasers, as well as high integration, high reproducibility, and low cost from optical fibers. This technology has emerged as a significant branch in the field of optical fiber sensing, thanks to the development of novel optical fiber structures and advanced fiber drawing technology. This chapter introduces the fundamental theory of FOFLs, and reviews various types of optical fiber microcavities, gain materials, and their sensing applications in ultrasensitive, rapid, high throughput, and disposable biodetection. With the emergence of new methods, materials and structures, FOFLs can provide new solutions for major disease diagnosis and environmental monitoring.