To contextualize this ecosystem, think about an investor that wants to short sell a security. The mechanics of short selling a security requires you first to borrow the security and then sell your borrowed security into the market. It is a two step process. You do not simply put through a trade order to short sell a stock like you do for purchasing a stock. The first step of the process to borrow the security takes place behind the scenes, and this is the part of the process that requires the prime broker. It's important to note that the prime broker acts as the middleman in the chain of events. The nature of their interaction as a middleman is on a principle basis, which means they are the one taking the counterparty risk to both the security lender and security borrower. It is not the case that they are simply connecting the security lender directly to the borrower, as is often the case in in other functions of a brokerage. This important risk relationship is why prime brokers charge margin on a securities lending arrangement. Margin is a metric which determines how much an investor is currently borrowing from the prime broker in the form of both borrowing securities and borrowing cash. With regards to securities lending, there is an important dynamic to note, which is that the prime broker never has net exposure to the market prices of the security being lent out. This is because they always are borrowing from their lenders. In other words, the inventory of securities left held at that prime brokerage by their clients. They are always borrowing the exact amount of the exact same securities that they are in turn lending out to their borrowers. This allows them to purely act as a middleman with no net exposure to either being net long or net short any securities. So no net exposure that would expose them to gains or losses from the fluctuation of the market price of these securities. Now that we've covered all of the risk relationships and flows of counterparties, it's worth covering the flow of fees in the ecosystem. The prime broker will attempt to borrow from the client accounts in their inventory that has the lowest cost to the broker. On some accounts like certain retail accounts, the cost to the prime broker is nothing. I.e. they do not give any borrow fees to that account when they borrow securities from them. The prime broker then takes that security and lends it out to the securities borrower who pays the prime broker a borrow fee. Think of it like an interest rate on borrowing money. The borrow fee is like the interest rate on borrowing a security. Alternatively, the securities could have come from the inventory of an institutional client's account. In this situation, there will often be an agreement in place for the institutional client to be paid their own borrow fee for allowing their securities to be lent. In this scenario, the prime broker's profits will be lowered to the spread between the borrow fee they pay to the security lender client versus the borrow fee. They are paid by the security borrower client. For example, if the borrow fee from the securities borrower is 3%, the prime broker will earn all 3% if they source the securities from a retail client account. But if the securities are lent by an institutional clients with a 2% lending fee in their contract, the profit to the prime broker will only be 1%. In addition to the borrow fees, another source of revenue for a prime broker would simply be an account maintenance fee. This would just be a regular flat fee charged to the clients for maintaining an open account with the prime broker and having access to their services and to keep the relationship active.