This paper gives an overview of fractional-N frequency synthesis with design perspectives focusing on ΔΣ fractional-N phase-locked loops (PLLs). Spur generation and nonlinearity issue in the ΔΣ fractional-N PLLs are discussed with hardware results. High-order ΔΣ modulation with embedded FIR filtering is considered for low noise frequency generation. Also, various architectures of finite-modulo fractional-N PLLs are reviewed for alternative low noise design.