A security deposit is a sum of money provided by one party (most commonly a tenant, licensee, or contracting counterparty) to secure performance of obligations and protect the other party against loss, damage, or default. In practice, security deposits are widely used in residential and commercial leases, construction contracts, and commercial agreements, and may take the form of cash, stakeholder-held funds, or equivalent security (for example, a rent deposit deed or escrow).In England and Wales and Northern Ireland, residential tenancy deposits are regulated, including by the Housing Act 2004 and associated tenancy deposit protection schemes. In Ireland, the Residential Tenancies Acts and Residential Tenancies Board guidance govern similar protections. In Scotland, tenancy deposits are governed by the Tenancy Deposit Schemes (Scotland) Regulations 2011.Security deposits are typically held subject to contractual terms specifying conditions for deduction, repayment, interest, and dispute resolution. They are distinct from guarantees and letters of credit but may operate alongside them. Case law in all jurisdictions addresses issues such as whether the deposit constitutes a trust, when set‑off or forfeiture is permissible, and the reasonableness of deductions on termination of the underlying relationship.