162 pages ; The conventional wisdom in systems and networking communities is that congestion in datacenter networks happens primarily within the network fabric (i.e., at network links and/or switches). This dissertation has three core contributions: (1) it presents a new problem of "host congestion''—congestion within the datapath between peripheral devices and compute/memory—and presents evidence of host congestion both in production datacenters and in experimental lab setups; (2) it builds an in-depth understanding of the root causes of host congestion, and of the impact of host congestion on application-level performance; and (3) it explores the implications of host congestion to the design of network protocols, network stacks and operating systems. We define host congestion in the context of networked applications as follows: the receiver-side host network interface card (NIC) receives data from the network at a rate faster than it can transfer it to compute/memory. This reduces the available NIC-to-memory bandwidth, resulting in queueing and eventual packet drops at hosts. We demonstrate that, even with state-of-the-art network protocols and network stacks, host congestion leads to significant degradation in throughput and orders-of-magnitude inflation in tail latency and a surprisingly large fraction of packet drops at the host, even when the access link bandwidth is far from fully utilized. We present evidence and characterization of the host congestion phenomenon for both large-scale production clusters (running Swift congestion control protocol with a userspace network stack), and in experimental testbeds (running DCTCP congestion congestion protocol with Linux network stack). Several recent studies have built upon our work to show that hardware-offloaded network stacks also suffer from similar or worse host congestion phenomenon. Host congestion, and resulting queueing and packet drops at the host, are new to the community. To that end, this thesis also builds an in-depth understanding of the root ...