In this talk, I focus on opportunities and challenges in conducting longitudinal studies. With data from children in the U.S., Beijing, and Hong Kong, I will illustrate how longitudinal studies have revealed important patterns in literacy learning. These studies highlight change over time, chart growth trajectories, are suggestive of some "causal" phenomena, and help to underscore and identify bidirectional associations. Longitudinal correlates of word reading and word writing (i.e., spelling) to be highlighted include speech perception, phonological sensitivity, morphological awareness, visual processing, general speed of processing, EEG signals, vocabulary knowledge, socio-economic status, and delayed copying. Using examples from different longitudinal studies spanning from one to twelve years, we will consider literacy development, as well as why it is important to distinguish learning from development in literacy acquisition. The talk will highlight a number of different published research studies with a view to understanding the importance of longitudinal work. I will also point out some "dos" and "don'ts" of longitudinal research given our work so far.