This chapter introduces cyber security researchers to key concepts in the data streaming and sketching literature that are relevant to Adaptive Cyber Defense (ACD) and Moving Target Defense (MTD). We begin by observing the challenges met in the big data realm. Particular attention is paid to the need for compact representations of large datasets, as well as designing algorithms that are robust to changes in the underlying dataset. We present a summary of the key research and tools developed in the data stream and sketching literature, with a focus on practical applications. Finally, we present several concrete extensions to problems related to ACD applications throughout this book, with a focus on improving scalability.