The article presents a tool and images to aid in identifying the stages of the mouse estrous cycle, which is crucial for timed mating and pseudopregnancy studies. The authors, from The Jackson Laboratory, describe the efficiency of visual observation and vaginal cytology in determining the stage of estrus. Visual observation is quick and requires no special equipment, making it suitable for identifying proestrus and estrus. However, strain differences, especially in coat color, can complicate accurate identification. Vaginal cytology, while more accurate, is labor-intensive and best used for all four stages of the estrous cycle (proestrus, estrus, metestrus, and diestrus). The article includes images of the vaginal opening at each stage for three mouse strains (black C57BL/6J, agouti CByB6F1/J, and albino BALB/cByJ) and introduces an estrous cycle identification tool to help users determine the stage based on cell types and proportions. The tool is particularly useful for those using vaginal cytology. The study emphasizes the importance of these methods in improving the efficiency of timed matings and pseudopregnancy studies.The article presents a tool and images to aid in identifying the stages of the mouse estrous cycle, which is crucial for timed mating and pseudopregnancy studies. The authors, from The Jackson Laboratory, describe the efficiency of visual observation and vaginal cytology in determining the stage of estrus. Visual observation is quick and requires no special equipment, making it suitable for identifying proestrus and estrus. However, strain differences, especially in coat color, can complicate accurate identification. Vaginal cytology, while more accurate, is labor-intensive and best used for all four stages of the estrous cycle (proestrus, estrus, metestrus, and diestrus). The article includes images of the vaginal opening at each stage for three mouse strains (black C57BL/6J, agouti CByB6F1/J, and albino BALB/cByJ) and introduces an estrous cycle identification tool to help users determine the stage based on cell types and proportions. The tool is particularly useful for those using vaginal cytology. The study emphasizes the importance of these methods in improving the efficiency of timed matings and pseudopregnancy studies.