Direct solar steam generation (DSSG) offers a promising, sustainable, and environmentally friendly solution to the energy and water crisis. In the past decades, DSSG has gained tremendous attention due to its potential applications for clean water production, desalination, wastewater treatment, and electric energy harvesting. Even though the solar–thermal conversion efficiency has approached 100% under 1 sun illumination (1 kW m−2) using various photothermal materials and systems, the optimization of the materials and system structure remains unclear because of the lack of evaluation methods in unity for the output efficiency. In this review, a few key concerns about different dimensional materials and systems that determine the characteristics of DSSG are explored. Quantitative analysis, including calculations and methods for the solar–thermal conversion efficiency, evaporation rate, and energy loss, is employed to evaluate the materials and systems from the point of view of ultimate utilization. This article focuses on the relationship between the system dimension and energy efficiency and notes opportunities for future system design and commercialization of DSSG. Direct solar steam generation requires multifunctional materials for powerful light absorption, well‐performing heat management, and sufficient water transport ability. The development and evolution of materials and system structures from 0D to 3D are summarized. Performance evaluations including efficiency, evaporation rate, and heat loss calculations, along with multifunctional utilizations, are highlighted.