CMOS image sensors have evolved significantly, offering a compact, low-power alternative to traditional charge-coupled device (CCD) sensors. This paper reviews recent advancements in CMOS image sensor technology, including passive pixel and active pixel sensors, which enable the realization of an electronic camera-on-a-chip. The integration of on-chip analog-to-digital converters, timing, and control circuits allows for a fully digital camera system with minimal power consumption and reduced system cost. CMOS image sensors, though historically less efficient than CCDs, have made significant strides in performance and functionality. The CMOS active pixel sensor (APS) now matches CCD performance with enhanced features, lower power consumption, and reduced system cost. The paper discusses various pixel implementations, including passive pixel, photodiode-type APS, and photogate-type APS, each with distinct advantages and applications. The paper also covers the development of on-chip analog signal processing and ADCs, which are crucial for improving image sensor performance. The roadmap for a camera-on-a-chip includes advancements in ADC technology, color filter arrays, and microlenses. The future of CMOS image sensors is promising, with continued improvements in scaling and integration, enabling more compact and efficient imaging systems. The paper highlights the potential of CMOS image sensors in a wide range of applications, from consumer electronics to spaceborne systems, and emphasizes their role in enabling low-cost, high-performance imaging solutions. The integration of CMOS technology with advanced signal processing and control circuits is paving the way for a new era of imaging systems that are more compact, efficient, and versatile than traditional CCD-based systems.CMOS image sensors have evolved significantly, offering a compact, low-power alternative to traditional charge-coupled device (CCD) sensors. This paper reviews recent advancements in CMOS image sensor technology, including passive pixel and active pixel sensors, which enable the realization of an electronic camera-on-a-chip. The integration of on-chip analog-to-digital converters, timing, and control circuits allows for a fully digital camera system with minimal power consumption and reduced system cost. CMOS image sensors, though historically less efficient than CCDs, have made significant strides in performance and functionality. The CMOS active pixel sensor (APS) now matches CCD performance with enhanced features, lower power consumption, and reduced system cost. The paper discusses various pixel implementations, including passive pixel, photodiode-type APS, and photogate-type APS, each with distinct advantages and applications. The paper also covers the development of on-chip analog signal processing and ADCs, which are crucial for improving image sensor performance. The roadmap for a camera-on-a-chip includes advancements in ADC technology, color filter arrays, and microlenses. The future of CMOS image sensors is promising, with continued improvements in scaling and integration, enabling more compact and efficient imaging systems. The paper highlights the potential of CMOS image sensors in a wide range of applications, from consumer electronics to spaceborne systems, and emphasizes their role in enabling low-cost, high-performance imaging solutions. The integration of CMOS technology with advanced signal processing and control circuits is paving the way for a new era of imaging systems that are more compact, efficient, and versatile than traditional CCD-based systems.