SIM_AGENT is a toolkit that arose out of a project concerned with designing an architecture for an autonomous agent with human-like capabilities. Analysis of requirements showed a need to combine a wide variety of richly interacting mechanisms, including independent asynchronous sources of motivation and the ability to reflect on which motives to adopt, when to achieve them, how to achieve them, and so on. These internal 'management' (and meta-management) processes involve a certain amount of parallelism, but resource limits imply the need for explicit control of attention. Such control problems can lead to emotional and other characteristically human affective states. In order to explore these ideas, we needed a toolkit to facilitate experiments with various architectures in various environments, including other agents. The paper outlines requirements and summarises the main design features of a Pop-11 toolkit supporting both rule-based and 'sub-symbolic' mechanisms. Some experiments including hybrid architectures and genetic algorithms are summarised.