APIs are the connective tissue of modern software, the interfaces through which applications, services, and partners exchange data. They are also, precisely because they expose functionality and data programmatically, a major and growing source of breaches. Securing them is now core to building anything serious.
The two foundational controls are distinct and both essential. Authentication verifies who is making a request, through keys, tokens, or credentials. Authorization verifies that the authenticated caller is allowed to do the specific thing they are asking for. A common and serious flaw is checking who someone is but not whether they should access a particular record, letting an authenticated user reach data that is not theirs by changing an identifier.
Two more basics prevent whole classes of abuse. Rate limiting caps how many requests a caller can make, blunting brute-force attacks, scraping, and denial-of-service attempts. Input validation ensures the data an API accepts is well-formed and safe, defending against injection and malformed-input attacks. Never trusting incoming data is a rule, not a preference.
Beyond these, good API security means encrypting traffic, minimizing the data each response returns, logging and monitoring for abuse, and keeping an inventory of the APIs you expose, since forgotten or undocumented endpoints are a frequent way in. Established guidance, like the OWASP API security list, catalogs the common pitfalls worth designing against from the start.
Verify the caller, the right, and the data.
This is general educational information, not specific security, compliance, or legal advice for your systems.