This document discusses Hidden Markov Models (HMMs). It begins by defining Markov chains as mathematical systems that undergo state transitions based only on the current state, not past states. It then explains that HMMs extend Markov models by making the observation of each state a probabilistic function of the state, so the underlying process is not directly observable. The document outlines the key elements of HMMs, different types, and issues in implementing them such as scaling, initial parameter estimates, training data size, and model choice. It concludes that HMMs can be used to develop voice-based user interfaces to help disabled people operate computers using speech.