XPath (XML Path Language) is a W3C standard query language designed for selecting nodes from XML documents. First published as a W3C Recommendation in November 1999 alongside XSLT 1.0 and XPointer, XPath treats an XML document as a tree of nodes and provides a path-based syntax for navigating that tree. The language has evolved through three major versions, each adding significant capabilities while maintaining backward compatibility.

XPath's data model represents an XML document as a tree with seven node types: root (document) node, element nodes, attribute nodes, text nodes, namespace nodes, processing instruction nodes, and comment nodes. This tree model is fundamental — every XPath expression is evaluated relative to a context node in this tree, and expressions navigate the tree using "axes" that define directional relationships between nodes.

XPath defines 13 axes that cover every possible navigational direction in the document tree. The most commonly used are child (direct children), descendant (all nested children), parent (immediate parent), ancestor (all parents up to root), following-sibling and preceding-sibling (nodes at the same level), and self (the current node). The abbreviated syntax that most developers learn — where / means child and // means descendant-or-self — maps to these axes. Understanding axes is essential for writing efficient XPath: //div is convenient but scans the entire document, while /html/body/div traverses a specific path and is much faster for large documents.

Node tests filter nodes along an axis by name or type. The expression //book selects element nodes named "book," while //text() selects text nodes and //@id selects attribute nodes named "id." Predicates in square brackets further filter results using conditions: //book[@price > 30] selects book elements where the price attribute exceeds 30, and //chapter[position() = 1] selects the first chapter element. Predicates can be nested and combined with Boolean operators (and, or, not), enabling complex selection logic.

XPath 1.0, the version most widely implemented in browsers and basic XML libraries, supports four data types: node sets, strings, numbers, and booleans. It provides a core function library including string functions (contains, starts-with, substring), numeric functions (sum, count, floor, ceiling), and node set functions (position, last, local-name). While powerful for basic querying, XPath 1.0 lacks support for regular expressions, date/time handling, and grouping operations.

XPath 2.0, published in 2007 as part of the XQuery/XSLT 2.0 family, dramatically expanded the language. It introduced a comprehensive type system based on XML Schema types, support for sequences (ordered collections replacing node sets), conditional expressions (if/then/else), quantified expressions (some/every ... satisfies), and range expressions (1 to 10). XPath 2.0 also added regular expression support, date/time arithmetic, and dozens of new functions.

XPath 3.0 (2014) and 3.1 (2017) further extended the language with higher-order functions (functions as values that can be passed to other functions), anonymous functions (lambda expressions), the arrow operator (=>) for pipeline-style function chaining, and support for maps and arrays. These additions transform XPath from a simple query language into a functional programming language capable of complex data transformations — though in practice, most developers use only XPath 1.0 features for basic node selection.