Photoacoustic imaging (PAI) is a very unique existence in biomedical imaging methods owing to the very different physical signals for excitation and detection. This "mixed" imaging mode breaks through the barrier in high-resolution optical imaging for deep tissue, providing a powerful platform technology for both biomedical research and clinical applications. However, PAI has not been widely used thus far. On the one hand, it is limited by its own mechanism, and on the other hand, there are some challenges in its practical application. This study discusses PAI technology based on the author's long-term scientific research experience and thinking in the field of PAI. Unlike the general research review, it does not discuss some of the important related technologies and application achievements in detail, instead it focuses on the historical development, connotation, and main challenges of PAI to foster the understanding of this technique and accelerate its implementations.