This document introduces network performance engineering and discusses three key concepts: 1) loss and delay accumulate along a network path, 2) the distribution of loss and delay is important, not just averages, and 3) loss and delay can be decomposed into geographic (G), serialisation (S), and variable contention (V) components. It argues this framework provides insights into broadband, LTE, SDN, and NFV that current approaches overlook by focusing on throughput over end-to-end quality of experience. Predictable Network Solutions and Martin Geddes Consulting aim to advance the practice of network performance engineering.