Embedded Networked Sensing involves untethered, networked devices tightly coupled to the physical world, to monitor and interact with it. Raw sensor observation can be annotated with semantic metadata to provide interpretation and context for it. In this paper, we discuss what, why and how of semantic sensor data and semantic sensor networks. Specifically, we explore (i) benefits of augmenting sensor data with semantics, (ii) domainspecific and spatio-temporal problems to be addressed, (iii) role of knowledge representation and reasoning (Semantic Web technology), and (iv) standardization efforts underway to make sensor-related data and sensor observations widely available. Categories and Subject Descriptors I.2.4 [Artificial Intelligence]: Knowledge Representation Formalisms and Methods General Terms Management, Design, Standardization Keywords Semantic Sensor Network, Semantic Web, Sensor Web, Ontology, Spatio-Temporal Reasoning.