Circular RNAs (circRNAs) are covalently closed RNA molecules that have been 
linked to various diseases, including cancer. However, a precise function and 
working mechanism are lacking for the larger majority. Following many different 
experimental and computational approaches to identify circRNAs, multiple circRNA 
databases were developed as well. Unfortunately, there are several major issues 
with the current circRNA databases, which substantially hamper progression in 
the field. First, as the overlap in content is limited, a true reference set of 
circRNAs is lacking. This results from the low abundance and highly specific 
expression of circRNAs, and varying sequencing methods, data-analysis pipelines, 
and circRNA detection tools. A second major issue is the use of ambiguous 
nomenclature. Thus, redundant or even conflicting names for circRNAs across 
different databases contribute to the reproducibility crisis. Third, circRNA 
databases, in essence, rely on the position of the circRNA back-splice junction, 
whereas alternative splicing could result in circRNAs with different length and 
sequence. To uniquely identify a circRNA molecule, the full circular sequence is 
required. Fourth, circRNA databases annotate circRNAs' microRNA binding and 
protein-coding potential, but these annotations are generally based on presumed 
circRNA sequences. Finally, several databases are not regularly updated, contain 
incomplete data or suffer from connectivity issues. In this review, we present a 
comprehensive overview of the current circRNA databases and their content, 
features, and usability. In addition to discussing the current issues regarding 
circRNA databases, we come with important suggestions to streamline further 
research in this growing field.