A two-state trajectory is a fluctuating signal made of on periods and off periods; an on period, and then an off period, and so on (see, Fig. 2). In most cases where this signal appears in applications in science, the trajectory is random; that is, the length of the on and off periods changes, and is a random quantity. There may be correlations in the trajectory; e.g., when we see a short off period and the next on period is relatively long (that is, long with a large probability), we say that there are off-on correlations. In principle, there are 4 independent types of correlations in two-state trajectories: on-on, on-off, off-on, and off-off. Two-state trajectories can be obtained from on-off kinetic schemes, RDFs, or any other stochastic equation of motion (with a clear on-off definition). In experiments from individual molecules, two-state trajectories are common, where from the trajectory we aim at finding the right model of the process.