Metamaterials offer the possibility to control and manipulate electromagnetic radiation. Spoof surface plasmon metamaterials are the focus of this Element of the Metamaterials Series. The fundamentals of spoof surface plasmons are reviewed, and advances on plasmonic metamaterials based on spoof plasmons are presented. Spoof surface plasmon metamaterials on a wide range of geometries are discussed: from planar platforms to waveguides and localized modes, including cylindrical structures, grooves, wedges, dominos or conformal surface plasmons in ultrathin platforms. The Element closes with a review of recent advances and applications such as Terahertz sensing or integrated devices and circuits.

The rise of the internet and mobile telecommunications has created the possibility of using large datasets to understand behavior at unprecedented levels of temporal and geographic resolution. Online social networks attract the most users, though users of these new technologies provide their data through multiple sources, e.g. call detail records, blog posts, web forums, and content aggregation sites. These data allow scholars to adjudicate between competing theories as well as develop new ones, much as the microscope facilitated the development of the germ theory of disease. Of those networks, Twitter presents an ideal combination of size, international reach, and data accessibility that make it the preferred platform in academic studies. Acquiring, cleaning, and analyzing these data, however, require new tools and processes. This Element introduces these methods to social scientists and provides scripts and examples for downloading, processing, and analyzing Twitter data.

Bioresorbable electronics that can dissolve away in aqueous environments and generate biologically safe products offer a revolutionary solution to replace the built-to-last electronics predominantly used in implanted devices and electronic gadgets. Their use can reduce the risk of surgical complications by minimizing the number of necessary surgeries, and prevent production of electronic waste by allowing rapid device recycling. This Element presents bioresorbable materials such as metals, polymers, inorganic compounds, and semiconductors that have been used to construct electronic devices, and analyzes their unique dissolution behaviors and biological effects. These materials are combined to yield representative devices including passive and active components and functional systems.

State capacity - the government's ability to accomplish its intended policy goals - plays an important role in market-oriented economic development today. Yet state capacity improvements are often difficult to achieve. This Element analyzes the historical origins of state capacity. It evaluates long-run state development in Western Europe - the birthplace of both the modern state and modern economic growth - with a focus on three key inflection points: the rise of the city-state, the nation-state, and the welfare state. This Element develops a conceptual framework regarding the basic political conditions that enable the state to take effective policy actions. This framework highlights the government's challenge to exert proper authority over both its citizenry and itself. It concludes by analyzing the European state development process relative to other world regions. This analysis characterizes the basic historical features that helped make Western Europe different. By taking a long-run approach, it provides a new perspective on the deep-rooted relationship between state capacity and economic development.

Institutions are the formal or informal 'rules of the game' that facilitate economic, social, and political interactions. These include such things as legal rules, property rights, constitutions, political structures, and norms and customs. The main theoretical insights from Austrian economics regarding private property rights and prices, entrepreneurship, and spontaneous order mechanisms play a key role in advancing institutional economics. The Austrian economics framework provides an understanding for which institutions matter for growth, how they matter, and how they emerge and can change over time. Specifically, Austrians have contributed significantly to the areas of institutional stickiness and informal institutions, self-governance and self-enforcing contracts, institutional entrepreneurship, and the political infrastructure for development.

This Element defends a reading of Kant's formulas of the moral law in Groundwork of the Metaphysics of Morals. It disputes a long tradition concerning what the first formula (Universal Law/Law of Nature) attempts to do. The Element also expounds the Formulas of Humanity, Autonomy and the Realm of Ends, arguing that it is only the Formula of Humanity from which Kant derives general duties, and that it is only the third formula (Autonomy/Realm of Ends) that represents a complete and definitive statement of the moral principle as Kant derives it in the Groundwork. The Element also disputes the claim that the various formulas are 'equivalent', arguing that this claim is either false or else nonsensical because it is grounded on a false premise about what Kant thinks a moral principle is for.

For advanced students of network data science, this compact account covers both well-established methodology and the theory of models recently introduced in the graphical model literature. It focuses on the discrete case where all variables involved are categorical and, in this context, it achieves a unified presentation of classical and recent results.

This snapshot of the current frontier of statistics and network analysis focuses on the foundational topics of modeling, sampling, and design. Primarily for graduate students and researchers in statistics and closely related fields, emphasis is not only on what has been done, but on what remains to be done.