BACKGROUND CONTEXT: Since the early 2000s, various expandable spinal fusion cages have been developed to facilitate less invasive procedures, however, expandable cages have often been evaluated as a homogeneous group, neglecting differences in shape, size, material, expandability and lordotic adjustability. This systematic review aimed to comprehensively survey the literature on expandable spinal fusion cages, discuss their differentiating factors, and identify gaps in the literature regarding these devices. PURPOSE: To demonstrate the range of design features included in expandable interbody devices and identify which of these features are associated with improved surgical outcomes. STUDY DESIGN: Systematic review. METHODS: The study followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines. An electronic search of MEDLINE and Embase using the search terms "lumbar" AND "fusion" AND ("expandable cage" OR "expandable interbody") including only English language articles that contained sufficient detail to correlate a specific expandable cage design to patient outcomes. Relevant elements, including device design parameters, patient population information, details of the intervention, comparison data, outcome variables, and the timeframe were extracted. Statistical analysis was conducted to correlate patient outcomes with different device features. RESULTS: While 387 different articles were initially identified, 49 met all the criteria for inclusion. Design differences contributed to disparate outcomes, with rectangular titanium cages featuring medial-lateral and vertical expansion and continuous lordotic adjustability being correlated with significantly improved patient-reported outcomes. The surgical approach and location were also found to be correlated with patient outcomes, indicating that confounding factors are present. CONCLUSIONS: We recommend that expandable cage technologies not be considered a homogenous group, as long-term outcomes likely are dependent upon specific design characteristics. Categorizing devices based on design features such as material composition, shape, vertical expandability, horizontal expandability, and restoration of segmental lordosis may allow for more rapid identification of device characteristics associated with better outcomes.