Infrastructure as a Service (IaaS) is a cloud computing model that delivers virtualized computing resources over the internet, including virtual machines, storage, networks, and operating systems. Unlike traditional on-premises infrastructure, IaaS operates on a pay-as-you-go basis, allowing organizations to provision and scale resources dynamically without purchasing physical hardware. Major providers include AWS (EC2, S3, VPC), Microsoft Azure (Virtual Machines, Blob Storage), and Google Cloud Platform (Compute Engine), each offering extensive global infrastructure spanning multiple regions and availability zones. The shared responsibility model defines clear boundaries: providers secure the underlying infrastructure while customers manage their data, applications, and access controls, making IaaS suitable for workloads ranging from development environments to enterprise production systems.