There are two types of systems for which time-resolved measurements are desired. The two types are distinguished by repeatability. Some systems are intrinsically very repeatable - these would probably include stretching of polymers within their elastic limit, and optical perturbation of systems which undergo a photocycle. Some systems such as explosions or process streams are intrinsically non-repeatable. There can also be some overlap between repeatable and non-repeatable events. If a laser used to trigger photocycles in a sample has a large shot-to-shot variation, then the repeatability will be poor. Combustion systems, even those where the reactants can be replaced from shot-to-shot, tend to have large variations. Photocycles of certain protein systems may have excellent reproducibility from shot-to-shot but photodegrade rapidly. The various step-scan and stroboscopic techniques can produce excellent results in cases where repeatability is good. They do not produce data in real time and very poor results are obtained when the system under study is unstable.