A health monitoring system for complex networked apparatus includes a number of neuro-fuzzy inference apparatuses feeding inference results into a data fusion hierarchy. At each level in the hierarchy, fuzzy inference is applied to generate a desired output signal by processing selected input signals in accordance with a knowledge base defining fuzzy membership functions and fuzzy inference rules defined in advance. The knowledge base includes alternative definitions of membership functions and/or inference rules. The apparatus selects which definition to use according to environmental or other conditions, and predetermined selection criteria.