C. elegans vulval development is an intensively studied example of animal organogenesis. A network of intercellular signaling, signal transduction, and transcriptional regulation underlies the precise formation of this organ, which is the connection between the hermaphrodite uterus and the outside of the nematode. A single cell of the somatic gonad, the anchor cell, organizes the development of the vulva from epidermal precursors as well as the physical connection of the epidermis with the uterus. WNT signaling acting via the HOX gene lin-39 renders six epidermal precursor cells competent to respond to other developmental signals. The anchor cell induces nearby epidermal precursor cells to generate vulval cells via an epidermal growth factor (EGF) signaling pathway. The precise pattern of vulval precursor cell fates involves the graded action of the EGF signaling and LIN-12 (Notch) mediated lateral signaling. EGF promotes the primary fate while LIN-12 promotes the secondary fate. Both EGF and LIN-12 prevent precursor cells from adopting the tertiary fate, which generates non-specialized epidermis. EGF-receptor and Notch signaling are antagonistic: EGF-receptor signaling leads to down-regulation of the Notch-like receptor LIN-12, while LIN-12 signaling induces negative regulators of EGF-receptor signaling such as MAP kinase phosphatase LIP-1 and the tyrosine kinase ARK-1. The primary precursor cell generates vulE and vulF mature vulval cells; the pattern of vulE and vulF cells requires an additional signal from the anchor cell as well as WNT signaling. The two secondary precursor cells generate vulA, vulB1, vulB2, vulC and vulD cells but in mirror symmetric polar patterns: ABCD and DCBA. The reversed polarity of the posterior secondary precursor cell lineage requires WNT signaling mediated by both Frizzled class and Ryk class WNT-receptors LIN-17 and LIN-18, respectively. A network of transcription factors controls the seven mature adult cell types; these include the LIM domain protein LIN-11, the Pax2/5/8 protein EGL-38, the zinc finger protein LIN-29, and the Nkx6.1/6.2 protein COG-1. The anchor cell also patterns nearby uterine cells, via the DSL ligand LAG-2 and LIN-12, to generate the four uv1 cells that form the tight connection with the vulva. This connection is initiated by the anchor cell, which invades between the vulF cells in a process analogous to invasive behavior of metastatic tumor cells. During this invasion process, the basement membranes between the gonad and body wall are degraded. The extensive information about vulval development in C. elegans has helped it become a paradigmatic case for identifying and studying a variety of regulatory pathways.