Smartphone-based biomedical devices have emerged as transformative tools at the intersection of engineering, biotechnology, and digital health. They are reshaping the delivery of diagnostic and monitoring capabilities in both clinical and nonclinical environments. The ubiquity, computational power, and connectivity of smartphones make them uniquely suited to function as diagnostic hubs. This enables the integration of microfluidics, electrochemical transducers, optical imaging systems, and artificial intelligence into compact, portable, and cost-effective platforms. This convergence has facilitated applications ranging from infectious disease detection, cancer biomarker analysis, to chronic disease management and global epidemiological surveillance. This chapter critically examines both the enabling technologies and the inherent constraints that govern smartphone-integrated biomedical devices. It explores fundamental challenges, including sensor precision, sample variability, environmental interference, and user-dependent operation, while mapping these limitations to emerging solutions in nanomaterial-based sensing, CRISPR-driven nucleic acid detection, federated learning architectures, and modular lab-on-chip platforms. Beyond technical considerations, the text analyzes the regulatory complexity of Software as a Medical Device (SaMD), the ethical implications of algorithmic transparency and informed consent, and the risks of digital inequities in global health contexts. Emerging research directions highlight the transformative role of smartphones in next-generation diagnostic ecosystems, where multi-parametric analysis, longitudinal health tracking, and digital twins will support personalized, predictive, and preventive medicine. The integration of artificial intelligence (AI)-driven closed-loop therapeutics, blockchain-secured data infrastructures, and scalable mobile networks positions smartphones as critical agents in reshaping both individual healthcare pathways and public health systems. By uniting principles from physics, chemistry, biology, computer science, and regulatory governance, this chapter presents a comprehensive and mechanistic account of smartphone-based biomedical technologies. It underscores their novelty as adaptive, distributed, and intelligent platforms that not only democratize diagnostics but also chart a path toward globally connected, anticipatory healthcare infrastructures.