Cross-section classification is one of the key concepts in the design of metallic structures. Current design specifications for aluminium alloys, such as Eurocode 9 (EC9), provide clear definitions and discrete design capacities for four different classes of cross-section. On the basis of substantial, recently generated experimental and numerical data on aluminium alloy cross-sections collected from the literature, the purpose of the present study is to re-evaluate the slenderness limits that define these classes. A total of approximately 900 relevant data points have been gathered, covering stub columns, simply supported beams and continuous beams; the cross-section types include square and rectangular hollow sections (SHS/RHS) with and without internal stiffeners, I-sections, channels and angles. The members were extruded from a variety of aluminium alloy tempers with a wide range of yield and ultimate strengths. Following analysis of the available data, the slenderness limits in EC9 have been re-assessed, and new slenderness limits in the EC9 framework are proposed. In addition, the full cross-section slenderness allowing for element interaction, which is utilised in the direct strength method (DSM) and the continuous strength method (CSM) has been considered as the slenderness parameter in a new classification framework. Corresponding slenderness limits, together with a compatible effective thickness formula for Class 4 sections, are proposed. The suitability of the proposed limits has been demonstrated for the conventional design methods in EC9, as well as the alternative methods in Annexes F and H of EC9.