"Fintech" and "regtech" are currently such popular buzzwords that one might assume every bank is implementing modern cloud-hosted systems for every business function at a breakneck pace. The reality is that substantially all of the software used at US banks is still "legacy" software running on-premise. At Alloy, we believe that a transformation is underway in financial software. But for fintech companies looking to sell software to banks, it is critical that they understand how legacy systems work and why they were implemented in the first place. We've spent the last two years creating and integrating regtech/fintech software, dealing with legacy infrastructure, and building new infrastructure for banks. Here's what we've learned.