Paper-based sensors have become suitable alternatives to analytical methodologies involving conventional instrumentation in a wide range of applications, being economically affordable, portable, disposable, and easy to use by untrained users. Colorimetry is the most widely employed detection method with paper-based sensors, and its increasing applicability, driven by the implementation of everyday IT and communications devices, is particularly remarkable. The present book chapter provides an overview of colorimetric paper-based analytical devices, focusing on the main principles of operation in paper-based sensors, strategies for color formation, fabrication techniques employed for their preparation, and strategies for quantitative analysis with both single and multiplexed devices. The chapter also provides a discussion of relevant applications of colorimetric paper-based sensors in food safety assessment, environmental monitoring, and point-of-care testing and diagnostics, and valuable contributions devoted to the development of colorimetric paper-based gas and vapor sensors, pH sensors, metal ion sensors, and biosensors are discussed. Challenging aspects and future prospects of colorimetric paper-based sensors are finally identified and discussed.