**1. Introduction**

Seed development is divided into three stages: embryogenesis, which includes embryo development, early maturation, or seed filling, which includes the accumulation of storage reserves; and late maturation, which includes seed desiccation and the transition to dormancy. After seed filling and desiccation, seed longevity increases up to 30-fold and places the embryo in a dormant state. The seed filling period accounts for between 10 and 78% of the total seed development period, but its importance during seed filling is still overlooked. Seed filling is a crucial stage for all seed plants, involving the synthesis of carbohydrates, lipids, and proteins, as well as the mobilization and accumulation of various components in the developing seeds. Although the metabolic pathways responsible for the synthesis of storage molecules are well known, their regulation is not well understood. Although seed filling is under genetic control, these developmental processes are influenced by the environmental factors, such as heat and drought stress. Therefore, environmental factors have major impacts on the qualitative and quantitative characteristics of seed development and

yield. Optimizing the rate and duration of seed filling could provide high and stable yields by reducing the potential negative effects of late maturation and maximizing the assimilation of metabolites formed by photosynthesis. Therefore, the rate and duration of seed filling are important determinants of seed quality and yield in many plants. In addition, understanding the complex processes during seed filling could help develop high-yielding cultivars under stress conditions.

In this chapter, we will attempt to summarize recent developments in the following areas: synthesis of storage reserves, genetic regulation, role of phytohormones, and effects of environmental factors to expand our understanding of these processes during seed filling. Given the emergence of new approaches to the study of seed filling and the tremendous growth of this topic in recent years, our discussion will inevitably be largely incomplete, and we apologize in advance to our colleagues.
