Novelty detection is essential for personal robots to continuously learn and adapt in open environments. This paper specifically studies novelty detection in the context of action recognition. To detect unknown (novel) human action sequences we propose a new method called background models, which is applicable to any generative classifier. Our closed-set action recognition system consists of a new skeleton-based feature combined with a Hidden Markov Model (HMM)-based generative classifier, which has shown good earlier results in action recognition. Subsequently, novelty detection is approached from both a posterior likelihood and hypothesis testing view, which is unified as background models. We investigate a diverse set of background models: sum over competing models, filler models, flat models, anti-models, and some reweighted combinations. Our standard recognition system has an inter-subject recognition accuracy of 96% on the Microsoft Research Action 3D dataset. Moreover, the novelty detection module combining anti-models with flat models has 78% accuracy in novelty detection, while maintaining 78% standard recognition accuracy as well. Our methodology can increase robustness of any current HMM-based action recognition system against open environments, and is a first step towards an incrementally learning system.@en