Wearable biosensing technologies are transforming healthcare by enabling continuous, real-time monitoring of physiological states at the point of care. Flexible microfluidics, particularly paper-based microfluidics, serve as critical interfaces between the body and soft electronics, enabling precise, capillary-driven, and non-invasive biofluid handling for real-time and clinically informative diagnostics. In this review, we discuss the fundamentals of paper-based microfluidics, highlighting critical considerations in material design, structural regulation, and interface engineering for precise capillary flow manipulation. We revisit fabrication techniques and key milestones in developing paper-based microfluidic devices, emphasizing innovative on-skin applications for wearable biofluid sampling, biosensing, and disease diagnostics. Finally, we outline persistent challenges that need to be addressed in the clinical translation of paper-based microfluidics for wearable healthcare and discuss future perspectives, including advances in paper materials engineering, integration with machine learning algorithms, and Internet-of-Things, to enable the next-generation personalized wearable healthcare solutions.