  Background: The field of software testing is growing and rapidly-evolving.
  Aims: Based on keywords assigned to publications, we seek to identify
predominant research topics and understand how they are connected and have
evolved.
  Method: We apply co-word analysis to map the topology of testing research as
a network where author-assigned keywords are connected by edges indicating
co-occurrence in publications. Keywords are clustered based on edge density and
frequency of connection. We examine the most popular keywords, summarize
clusters into high-level research topics, examine how topics connect, and
examine how the field is changing.
  Results: Testing research can be divided into 16 high-level topics and 18
subtopics. Creation guidance, automated test generation, evolution and
maintenance, and test oracles have particularly strong connections to other
topics, highlighting their multidisciplinary nature. Emerging keywords relate
to web and mobile apps, machine learning, energy consumption, automated program
repair and test generation, while emerging connections have formed between web
apps, test oracles, and machine learning with many topics. Random and
requirements-based testing show potential decline.
  Conclusions: Our observations, advice, and map data offer a deeper
understanding of the field and inspiration regarding challenges and connections
to explore.
