Logging and auditing is an important system facility for monitoring correct system operation and for detecting potential security problems. We present an architecture for implementing user-level auditing monitors which: (i) does not require superuser privileges; (ii) makes it simple to create user defined monitors which are transparent; and (iii) provides security guarantees such as mandatory and reliable monitoring while maintaining confidentiality of setuid processes. We avoid problems of self-referential monitoring. Monitor use policies can be specified to increase flexibility. We show that our framework can be tailored so that it is very efficient with low overhead on macro and micro benchmarks. This demonstrates that it is feasible to make use of arbitrary and programmable user-level monitors for system security and auditing applications.