Organic-inorganic halide perovskite solar cells (PSCs) have achieved remarkable power conversion efficiency (PCE) of up to 26.1%, comparable to traditional silicon solar cells. However, their PCE remains below the Shockley-Queisser limit, and they suffer from poor intrinsic stability under ambient conditions. Recent studies have demonstrated that both the PCE and stability of PSCs can be significantly enhanced through careful material selection and interface engineering during fabrication. Interface engineering has emerged as a promising strategy to improve PCE and intrinsic stability by facilitating efficient carrier extraction at interfaces, reducing trap states, and mitigating undesirable degradation pathways. This chapter will discuss the following aspects like energy level alignment, interfacial defects, carrier dynamics, mitigation of ion migration, and obstacle for moisture penetration and electrode diffusion. Additionally, the discussion covered various types of materials used as interlayers at different interfaces, including electrode/ETL, ETL/perovskite layer, perovskite layer/HTL, and HTL/electrode interfaces. Furthermore, it explored interfacial engineering strategies for flexible perovskite solar cells (PSCs) and provided a future outlook for this rapidly evolving field.