This chapter reviews recent research investigating children’s Spontaneous Focusing On Numerosity (SFON) and considers the role it might play in the development of counting and arithmetical skills. SFON refers to a process of spontaneously (i.e. not prompted by others) focusing attention on the exact number of a set of items or incidents. This attentional process triggers exact number recognition and using the recognized exact number in action. The chapter describes how SFON tendency can be assessed, and suggests the measures of it to be indicators of the amount of a child’s self-initiated practice in using exact enumeration in his or her natural surroundings. The studies show that SFON tendency in early childhood is positively and domain-specifically related to the development of numerical skills up to the end of primary school. Promoting SFON tendency could be a potential way of preventing learning difficulties in mathematics.