We interviewed four UK companies from different verticals about the growth they're witnessing in mobile usage, traffic and transactions, as well as how these trends are shaping decisions about website development. Domino's Pizza maintains separate sites for different devices, while Rentalcars.com, Feelunique and Tesco Bank have embraced a responsive web design (RWD) approach. Here they outline the challenges and opportunities afforded by today's multi-channel environment.