Reading is a complex process that requires the integration of several cognitive variables. Three cognitive components are involved in the assessment of reading abilities in the tonal language of Thai, which is similar to Chinese and is one of the major languages of Southeast Asia. This chapter presents an overview of reading assessment of the Thai language, focusing on the cognitive components of the reading process. A case study assessing the reading abilities of 357 fourth-grade Thai-speaking students was used to illustrate the reciprocal relationships between cognitive components and reading abilities. The cognitive components include decoding, morphological awareness and rapid naming, while reading abilities include reading comprehension and word recognition. Two-wave cross-lagged panel models were used to ascertain the directionality of reading development over a period of one year. Results of the case study showed a bidirectional association between cognitive components and reading ability over time. These results indicate that reading development in the Thai language benefits not only from traditional bottom-up and top-down processing but also from an interactive view of reading.