Freezing of gait (FOG) is a common and debilitating gait impairment in Parkinson's disease. Further insight in this phenomenon is hampered by the difficulty to objectively assess FOG. To meet this clinical need, this paper proposes a motion capture-based FOG assessment method driven by a novel deep neural network. The proposed network, termed multi-stage graph convolutional network (MS-GCN), combines the spatial temporal graph convolutional network (ST-GCN) and the multi-stage temporal convolutional network (MS-TCN). The ST-GCN captures the hierarchical motion among the optical markers inherent to motion capture, while the multi-stage component reduces oversegmentation errors by refining the predictions over multiple stages. The proposed model was validated on a dataset of fourteen freezers, fourteen non-freezers, and fourteen healthy control subjects. The experiments indicate that the proposed model outperforms state-of-the-art baselines. An in-depth quantitative and qualitative analysis demonstrates that the proposed model is able to achieve clinician-like FOG assessment. The proposed MS-GCN can provide an automated and objective alternative to labor-intensive clinician-based FOG assessment. I. INTRODUCTION C OMPARED to other neurological disorders, Parkinson's disease (PD) has a fast growing prevalence, doubling every 20-30 years [1]. Freezing of gait (FOG) is a common and debilitating gait impairment of PD. Up to 80% of the people with Parkinson's disease (PwPD) will develop FOG during the course of the disease [2], [3]. FOG leads to sudden blocks in walking and is clinically defined as a "brief, episodic absence or marked reduction of forward progression of the feet despite the intention to walk and reach a destination" [4]. The PwPD themselves describe freezing of gait as "the feeling that their feet are glued to the ground" [5]. Freezing episodes most frequently occur while traversing under environmental constraints, during emotional stress, during cognitive overload by means of dual-tasking, and when initiating gait [6], [7]. Though, turning hesitation was found to be the most frequent trigger of FOG [8], [9]. Subjects with FOG experience more anxiety [10], have a lower quality of life [11], and are at a much higher risk of falls [12], [13], [14], [15], [16]. Given the severe adverse effects associated with FOG, there is B. Filtjens and B. Vanrumste are with the