State estimation is a problem that occurs in many fields. A very well known solution to the state estimation problem occurs when the system satisfies a linear Markovian model and disturbances have a Gaussian distribution. This leads to the celebrated Kalman filter. In this seminar, we will study the problem of state estimation in the presence of constraints. Two types of constraints will be considered, namely integral and finite alphabet constraints on disturbances and finite alphabet constraints on observations. We will show how rolling horizon optimization techniques and Markov Chain Monte Carlo methodologies can be utililzed to solve these problems. The talk will be illustrated by a number of examples including control over network communications systems and air/fuel ratio control in automobile engines