Chapter 4 examines the core principles behind designing effective EdTech, emphasizing that technological innovation must be grounded in how children learn, feel, and act. The chapter argues that socio-emotional learning (SEL) is not an optional add-on but a foundational component of impactful digital tools, as children’s motivation, confidence, and emotional regulation shape every learning interaction. It discusses how thoughtfully designed EdTech can foster these skills through supportive feedback, collaborative activities, and features that nurture perseverance and curiosity. A central focus is student agency, that is the ability of learners to make choices, influence their learning pathways, and engage meaningfully with content. The chapter demonstrates how design choices can either amplify agency or unintentionally limit it. Drawing on research examples, it outlines practical design principles that integrate SEL, agency, and cognitive science to create tools that genuinely enhance learning. The conclusion calls for intentional, evidence-aligned design as the backbone of responsible EdTech development.