Paper-based sensors exploiting the advantages of paper can replace traditional substrate materials for building sensors which are simple to manufacture, inexpensive, easy-to-operate, portable and disposable. From clinical diagnostics and agriculture to wearable devices and environmental and food science, paper-based sensors have enormous potential for applications, especially in resource-constrained settings. In this review, we summarize and discuss the latest research concerning paper-based sensors for applications in diagnostics, food, environment, agriculture and wearable sensing. We analyze the current challenges, offer potential solutions and highlight the opportunities.