  Following the first two annual intensity mapping workshops at Stanford in
March 2016 and Johns Hopkins in June 2017, we report on the recent advances in
theory, instrumentation and observation that were presented in these meetings
and some of the opportunities and challenges that were identified looking
forward. With preliminary detections of CO, [CII], Lya and low-redshift 21cm,
and a host of experiments set to go online in the next few years, the field is
rapidly progressing on all fronts, with great anticipation for a flood of new
exciting results. This current snapshot provides an efficient reference for
experts in related fields and a useful resource for nonspecialists. We begin by
introducing the concept of line-intensity mapping and then discuss the broad
array of science goals that will be enabled, ranging from the history of star
formation, reionization and galaxy evolution to measuring baryon acoustic
oscillations at high redshift and constraining theories of dark matter,
modified gravity and dark energy. After reviewing the first detections reported
to date, we survey the experimental landscape, presenting the parameters and
capabilities of relevant instruments such as COMAP, mmIMe, AIM-CO, CCAT-p,
TIME, CONCERTO, CHIME, HIRAX, HERA, STARFIRE, MeerKAT/SKA and SPHEREx. Finally,
we describe recent theoretical advances: different approaches to modeling line
luminosity functions, several techniques to separate the desired signal from
foregrounds, statistical methods to analyze the data, and frameworks to
generate realistic intensity map simulations.
