Interior design is a multidisciplinary field in which the fine arts intersect with engineering sciences and environmental psychology. Its modern concept is no longer limited to visual appearance or decorative features; rather, it has evolved into a comprehensive analytical discipline that seeks to connect architectural spaces with human physiological and psychological responses. The functional philosophy of contemporary design is grounded in the principle that “form follows function,” which is closely associated with modernist movements and the Bauhaus school. Within this approach, spatial design begins with the study of movement and the organization of circulation paths to facilitate everyday activities and improve efficiency. Interior space is a dynamic environment that influences users’ comfort and psychological responses. A balanced arrangement of furniture and architectural elements can help reduce stress and promote a sense of stability. Both natural and artificial lighting play essential roles in highlighting the aesthetic qualities of surfaces and design elements, while also influencing human circadian rhythms and the sleep–wake cycle. Materials and finishes work together to create a multidimensional sensory experience. Textures and material diversity give interior spaces visual and tactile depth, enriching the user’s everyday experience. The use of natural materials, such as wood and stone, alongside plants and natural light, supports the principles of biophilic design, which draws on the innate human tendency to connect with nature. These approaches can contribute positively to well-being and psychological balance within enclosed environments. Contemporary color strategies also rely on an understanding of the psychological and perceptual effects of color. Warm colors may encourage a sense of energy and interaction, while certain cool colors can help create calmer environments that support concentration. However, these effects vary according to cultural context and the intended use of the space. Successful interior design extends beyond aesthetic and functional considerations to include climate-responsive strategies and energy efficiency. These can be achieved through the effective use of natural lighting and ventilation, appropriate material selection, and acoustic treatments that reduce noise and improve the quality of indoor environments in residential and office spaces. Such considerations emphasize the importance of balancing human needs with environmental responsibility and sustainability. Recent technological developments have introduced new concepts, particularly smart design, which integrates automation and digital control systems into interior environments to improve comfort and operational efficiency. Universal design has also become increasingly important, addressing the needs of diverse groups within society, including older adults and people with disabilities, to ensure accessibility and ease of use. In addition, flexible and multifunctional furniture offers an effective design solution for maximizing limited spaces, particularly in urban environments where efficient spatial planning is increasingly necessary. The success of interior design is also closely linked to post-occupancy evaluation, a systematic method for assessing how well a space performs after it has been occupied. This process examines whether functional and environmental objectives have been achieved while analyzing users’ actual behavior and needs. Its findings can inform design improvements and contribute to the development of higher-quality indoor environments in the future. In conclusion, interior design goes beyond an applied skill or an aesthetic practice to become an integrated human-centered and engineering discipline that combines functionality, beauty, technology, and sustainability. It reflects the development of societies and their awareness of the importance of the environments in which people live and work. Ultimately, it expresses a vision of creating spaces that enhance comfort, efficiency, and aesthetic quality in everyday life.