The use of wireless sensor network testbeds can significantly improve the efficiency and accuracy of the assessments of algorithms, protocols and applications by employing real physical nodes in an experimental environment. As such, it is very important to optimize the selection and use of existing technologies and resources to ensure that test platforms are characterized by high availability, scalability, and interactivity. In this paper, we summarize a number of current problems and challenges experienced in heterogeneous wireless sensor network testbeds in the processes of their design, implementation, and experimentation. We then analyze in detail the existing solutions and research methods employed in heterogeneous testbeds, and compare the technologies and characteristics of some representative testbeds. Finally, we identify outstanding research issues with respect to wireless sensor network testbeds.