At the end of 2019, about 4.1 billion people on earth were using the internet. Because people entrust their most intimate and private data to their devices, the European legislation has declared the protection of natural persons in relation to the processing of personal data as a fundamental right. In 2018 23 million people worldwide, having the responsibility of implementing data security and privacy, were developing software. However, the implementation of data and application security is a challenge, as evidenced by over 41 thousand documented security incidents in 2019. Probably the most basic, powerful, and frequently used tools software developers work with are Application Programming Interfaces (APIs). Security APIs are essential tools to bring data and application security into software products. However, research results have revealed that usability problems of security APIs lead to insecure API use during development. Basic security requirements such as securely stored passwords, encrypted files or secure network connections can become an error-prone challenge and in consequence lead to unreliable or missing security and privacy. Because software developers hold a key position in the development processes of software, not properly operating security tools pose a risk to all people using software. However, little is known about the requirements of developers to address the problem and improve the usability of security APIs. This thesis is one of the first to examine the usability of security APIs. To this end, the author examines to what extent information flows can support software developers in using security APIs to implement secure software by conducting empirical studies with software developers. This thesis has contributed fundamental results that can be used in future work to identify and improve important information flows in software development. The studies have clearly shown that developer-tailored information flows with adapted security-relevant content have a positive influence on the correct implementation of security. However, the results have also led to the conclusion that API producers need to pay special attention to the channels through which they direct information flows to API users and how the information is designed to be useful for them. In many cases, it is not enough to provide security-relevant information via the documentation only. Here, proactive methods like the API security advice proposed by this thesis achieve significantly better results in terms of findability and actionable support. To further increase the effectiveness of the API security advice, this thesis developed a cryptographic API warning design for the terminal by adopting a participatory design approach with experienced software developers. However, it also became clear that a single information flow can only support up to a certain extent. As observed from two studies conducted in complex API environments in web development, multiple complementary information flows have to meet the extensive information needs of developers to be able to develop secure software. Some evaluated new approaches provided promising insights towards more API consumer-focused documentation designs as a complement to API warnings.