The 2025 volume of the Vienna Yearbook of Population Research focuses on the role of population inequality in demographic research, particularly, on the interplay between population diversity and social inequality. Besides classical markers of heterogeneity in individual behavior, such as gender, age, education, family status, migration background, urban-rural residence and socio-economic status, other sources of inequality are covered in the volume. They include marginalized populations, such as homeless people, generational and spatial factors as well as emerging trends, such as digitalization. Understanding population inequality is key for modeling population developments and projecting them into the future. Equally important is to understand how and why different types of inequality arise and evolve, and what policy challenges they impose for socio-economic development, welfare systems and social cohesion.