I will review the notion of (continuous) 2-group global symmetries, which mix ordinary and higher-form global symmetries, and explain why they naturally arise in many 6d gauge theories. Most 6d SCFTs and little string theories have weakly-coupled infrared phases with gauge fields. While non-trivial 2-group symmetries are often present in little string theories, I will explain why they can never arise in unitary SCFTs. This observation can be used to establish a previously conjectured algorithm for computing ’t Hooft anomalies of such SCFTs. Finally, I will use this understanding to shed light on the a-theorem in six dimensions. In particular, I will show that all unitary 6d SCFTs have positive a-anomaly.