In recent years, rapidly changing working conditions have stimulated employees to invest more time and effort in work. These changes call for a better understanding of how heavy work investment (i.e., a strong focus on the task at hand and a high level of dedication to work) impacts employees and organizations. The aim of this chapter is to discuss heavy work investment and its outcomes in terms of work–family balance among Japanese dual-earner couples. In the first part of the chapter, I introduce two different types of heavy work investment (workaholism and work engagement) and describe correlates of them with well-being and job performance. In the second part, I introduce working conditions and family structures in Japan. In the third part, I refer to the spillover–crossover model as a conceptual framework and then move to a general overview of empirical studies conducted in Japan. Finally, I discuss future directions of work–life balance research in terms of the spillover–crossover model.