The completion of plant genome sequences and advances in DNA microarray technology have contributed to the accumulation of a vast number of plant gene expression datasets. Co-expression analyses using such datasets can be used to predict the functions of many plant genes. A network approach has been incorporated into co-expression analysis to visualize gene-to-gene functional relatedness (co-expression network analysis). Applying of this analysis to plant gene expression datasets has led to the accumulation of large quantities of information on plant gene function. Plant gene expression datasets and genome-level information obtained using co-expression analyses in plants can be retrieved from various types of databases. Here, we summarize practical approaches for detecting co-expressed genes in plants and review recent progress in plant co-expression analyses.