Longitudinal research refers to research that is done over a long period of time. For example, interviewing participants at regular interviews through their childhood, in order to see development.
An advantage of using longitudinal research is that researchers can genuinely see and record social change over significant time periods.
One of the main disadvantages of this method is the practical issue of cost, meaning funding agencies who pay for research may be unwilling to commit to an expensive project whose results will be unknown for many years.
A popular (rather than academic) example of a longitudinal study would be the television programme 7 Up.