Thermoplastic stormwater chambers have been successfully used on many projects to meet current federal regulations controlling discharges from newly developed sites. These structures are commonly located under parking lots or roadways and subjected to vehicular loads on a regular basis. Given this application, it is logical to design these structures for loads specified in the AASHTO LRFD Specifications for Highway Bridges, which provide design procedures for thermoplastic culvert pipe for use under roadways. However, experience has shown that many drainage engineers are not familiar with the AASHTO Specifications and have difficulty evaluating product manufacturer's claims relative to AASHTO requirements. This paper presents the key aspects of the AASHTO specifications that should be applied to stormwater chamber design, such as determining applicable loads, load factors, design of profile sections, and consideration of time dependent properties. Also addressed are design matters for chambers not directly addressed by AASHTO. Of particular importance in developing unique structures is the validation of designs and chamber safety levels through full scale testing.