A key question in reproductive physiology is how women retain a sufficient supply of oocytes to support regular ovulation for their reproductive lifespan, until the menopause at around 50 years of age. It is generally accepted that female mammals are born with a finite stock of oocytes. At, or soon after, birth each oocyte in the ovary is enveloped by one or more layers of specialized somatic cells (granulosa cells, GCs) to form a structure known as a follicle. The majority of follicles in the ovary are at the primordial (resting or quiescent) stage of development, where the oocyte is small and surrounded by a single layer of flattened GCs. From birth onwards, a steady trickle of primordial follicles initiate growth, with the oocyte growing in size and the GCs becoming cuboidal and starting to proliferate. A small proportion of growing follicles eventually ovulate, although the majority die and the population of follicles within the ovary declines steadily with age. The rate at which follicles initiate growth has to be carefully regulated, as mammals have to maintain a sufficient supply of primordial follicles to last their reproductive lifespan, while providing enough growing follicles for regular ovulation. Deviations in the rate at which follicles initiate growth will have a significant bearing on fertility, the age of the menopause and disorders of ovulation in women, such as primary ovarian failure and polycystic ovary syndrome (PCOS). Despite its importance in controlling reproductive lifespan, the mechanism regulating initiation of follicle growth remains unclear. Studies of follicle development in mice lacking specific genes, and in ovaries or pieces of ovary cultured under different conditions in vitro, suggest that early follicle development is mainly regulated by local growth factor signals. However, while a number of growth factors have been implicated in early follicle development, particularly members of the Transforming Growth Factor-beta superfamily, the source and identity of the key factor or factors that signal a follicle to start growing are still unknown. We have recently developed an alternative strategy for identifying the source and action (stimulatory or inhibitory) of local signals by analysing and quantifying the spatial distribution of primordial and growing follicles within the ovary. This was stimulated by the qualitative observation that primordial follicles are generally located in the outer cortex of the ovary, often in clusters, while growing follicles are found towards the medulla, leading us to speculate that primordial follicles produce an inhibitor. We tested this quantitatively by measuring the XY coordinates of follicles within histological sections of mouse ovaries, and calculating the distances between every follicle in the section. Follicles were grouped according to the number of resting and growing follicles in their immediate vicinity. The key finding was that follicles were significantly less likely to be growing if they had one or more resting primordial follicles close by, suggesting that primordial follicles do indeed produce an inhibitor. Further analysis of the spatial distribution of resting and growing follicles suggested that this inhibitor is diffusible, and furthermore that growing follicles produce a stimulatory factor. These studies have provided insight into the local ovarian signals that determine whether a follicle is quiescent or starts to grow and provide us with important leads in the investigation of candidate growth factor pathways. Such studies will provide a better understanding of, and treatments for, ovarian disorders and we have recently applied this approach to the study of human ovaries. This approach could be extended to other branches of developmental and cell biology to study signalling within tissues and cells.