The RIG-I-like family of receptors is positioned at the front line of our innate cellular defence system. RIG-I detects and binds to viral duplex RNA in the cytoplasm of both immune and non-immune cells, and initiates the induction of type I interferons and pro-inflammatory cytokines. The mechanism of RIG-I activation by dsRNA involves a molecular rearrangement proposed to expose the N-terminal pair of CARDs enabling an interaction with IPS-1, thereby initiating downstream signalling. dsRNA is particularly stimulatory when longer than 20 bp, potentially through allowing binding of more than one RIG-I molecule. An intense international effort has resulted in the determination of the structures of a range of RIG-I truncation mutants. Here, we characterize full-length RIG-I and RIG-I subdomains combined with a stimulatory 29mer dsRNA using MALS and SEC-coupled SAXS. We present the solution structure of the human apo-RIG-I and observe that upon binding of RIG-I to dsRNA in a 2:1 ratio, the complex becomes highly extended and flexible which explains mechanism for viral RNA stimulated signalling by RIG-I.