Network programmability has provided an effective approach to enable innovation in network systems. By replacing static, expensive middleboxes with equivalent pieces of software implementing the same functionality, operators can significantly reduce their CAPEX/OPEX expenditures, and engineers can rapidly design, test and deploy novel architectures and services, thus reducing the time-to-market for network applications. However, the flexibility provided by software solutions comes at a cost: purposespecific hardware has the clear advantage of optimized performance (i.e., throughput, latency) with respect to pure software-based solutions. The introduction of software acceleration techniques represented an essential step towards the increasing popularity of the SDN/NFV paradigm shift, by reducing the performance gap between hardware-based and software-based systems. Thanks to such techniques, modern software-networking solutions can operate at multi-10Gbps rate (up to hundreds of Gbps) on commodity servers equipped with regular COTS components. In this chapter, we cover the aspects related to software acceleration techniques in a bottom-up fashion: we first provide an overview of high-speed software networking on COTS architectures, and we then explore the evolution of softwarized networking by focusing on performance acceleration and the design space for high-speed programmable networks.