Intrinsic rhythmic processes govern a wide range of biological activities in all organisms, from microscopic bacteria to large mammals. In algae, circadian rhythms play a vital role in regulating daily life processes—from controlling buoyancy and movement in green algae to timing cell division, managing UV sensitivity, and guiding starch accumulation. These rhythms enhance survival by aligning internal functions with environmental changes. Within the cell, they influence crucial activities such as transcription and DNA repair in the nucleus, responding to external cues that are interpreted by a central oscillator and expressed through targeted gene regulation. Circadian rhythms can be traced back to cyanobacteria, the earliest known prokaryotes with a circadian clock. Core clock components, including PRR and RVE genes, are conserved across the green lineage. Among the most valuable models for studying circadian rhythms is Chlamydomonas reinhardtii, owing to its well-regulated traits such as phototaxis, chemotaxis, and cell adhesion. Its simple life cycle, ease of mutant isolation, and expanding genetic tools further enhance its research utility. Comparative studies with other algae have identified key circadian genes in Chlamydomonas, particularly the ROC protein family (ROC15, ROC40, ROC55, ROC66, ROC75, and ROC114). Notably, C. reinhardtii shares homologous genes with Arabidopsis thaliana, making it a crucial system for circadian studies. Beyond C. reinhardtii, various other algal systems have expanded our understanding of circadian rhythms.