htop is a cross-platform interactive process viewer and system monitor written in C on ncurses. It enumerates and displays processes by reading OS kernel interfaces — primarily /proc on Linux, sysctl/kvm/kstat on the BSDs and Solaris, libproc/mach on macOS, and PCP (Performance Co-Pilot) metrics for the pcp-htop variant — and renders a scrollable process table alongside a header of pluggable meters (CPU, memory, swap, disk I/O, network I/O, ZFS ARC, GPU utilization, pressure stall, battery, sensors, and more). Beyond display, it supports process management actions: kill, renice, set CPU affinity, scheduling and I/O priority, spawning strace/lsof child processes, plus tree view, sorting, filtering, incremental search, and persistent configuration in htoprc. Architecturally this is a monolithic C application with emulated object orientation: an Object base class with vtable-style Class function tables, generic containers (Hashtable with Robin Hood open addressing, Vector with ownership semantics), a RichString wide-character rendering layer, reusable Panel/Meter widget classes, and a ScreenManager event loop. Each supported OS lives in a backend directory (linux/, darwin/, freebsd/, openbsd/, netbsd/, dragonflybsd/, solaris/, pcp/, unsupported/) that implements the Platform/Machine/ProcessTable/Process contract. Entry points are htop.c and pcp-htop.c, which route through CommandLine_run() into ScreenManager_run(). The build uses GNU autotools with optional features (hwloc, libsensors, libnl, libcap, libunwind, PCP), and CI runs the binary headlessly under sanitizers plus a Lua/rote terminal-emulation test suite. Maintenance is genuinely active: recent commits, a CodeQL workflow, address/UB sanitizer jobs, valgrind suppressions, and a ChangeLog showing security fixes (CVE-2024-37676) and steady feature work through 3.6.0-dev. The honest caveats are platform fragility: Solaris and OpenBSD have TODO stubs for disk/network I/O, NetBSD uses deprecated proplib compatibility aliases, /proc and DRM fdinfo parsers are sensitive to kernel format changes, and there is visible copy-paste divergence across BSD backend field arrays. The core Linux path is current and well-maintained; the tail platforms lag, which keeps overall technical drift at medium rather than low.