In this thesis, I introduce Lazy Asynchronous I/O (LAIO), a new API for performing I/O that is well-suited but not limited to the needs of high-performance, event-driven servers. In addition, I describe and evaluate an implementation of LAIO that demonstrably addresses certain critical limitations of the asynchronous and non-blocking I/O support in present Unix-like systems. LAIO is implemented entirely at user-level, without modification to the operating system's kernel. It utilizes scheduler activations. Using a micro-benchmark, LAIO was shown to be more than 3 times faster than AIO when the data was already available in memory. It also had a comparable performance to AIO when actual I/O needed to be made. An event driven web server (thttpd) achieved more than 38% increase in its throughput using LAIO. The Flash web server's throughput, originally achieved with kernel modifications, was matched using LAIO without making kernel modifications.