The Second Demographic Transition (SDT) predicts a sustained movement toward significantly reduced fertility rates and diverse family structures. This shift is primarily driven by an inexorable and irreversible change in societal attitudes and norms, emphasizing increased individual freedom and self-realization. A central element of this transition is the substantial decline in fertility rates, descending from slightly above the replacement level of 2.1 births per woman to a notably lower level. Consequently, this decline in fertility leads to a transformation in household structures, which encompass the arrangement of roles, power dynamics, statuses, and relationships within families. The specific configuration of household structures is influenced by factors such as the socioeconomic background, societal patterns, and the degree of urbanization. Ideational change, involving alterations in ideas, beliefs, values, and cultural norms, shapes individual perceptions, thoughts, and behaviors, thereby influencing broader societal dynamics. This collective shift in mindset acts as a pivotal force driving transformations across social, economic, and political realms, impacting the evolution of society. This chapter explores the core elements of SDT and the broader influence of ideational change on societal development.