[Abstract] This paper describes lunar lander design trade studies performed by Boeing using the Altair concept mission definitions. Complex and multi-layered design interrelationships surfaced during this effort. Trade spaces were developed for the Ascent Module (AM) and Descent Module (DM) to address these interrelationships. Figures of merit (FOMs) were developed to rate the trade study candidates in various performance categories. The trade study discussion provides the rationale and methodology for the trade studies, including the set of possible trade candidates, the candidates that were excluded, and a description of the FOMs. The AM and DM trade studies were performed to optimize the layout and FOMs of candidate AM and DM designs. Promising AM and DM concepts were rated side-by-side using the FOMs. The DM structural concept trade study provided inputs to FOM ratings for candidate DM concepts and guidance and design rules for the layout of integrated lunar lander vehicles. Trade study results were incorporated into four integrated lunar lander vehicle concepts: A lander with four liquid oxygen (LO2) tanks and four liquid hydrogen (LH2) tanks, a lander with a single LO2 tank and four LH2 tanks, a lander with a single LH2 tank and four LO2 tanks, and a lander with single LH2 and LO2 tanks separated by a common bulkhead. Structural concepts for the four lander vehicles range from a full space truss for the eight-tank design to a full shell for the two-tank design, with a mixture of truss and shell elements for the two five-tank designs. The four lander vehicles incorporate two promising candidates from the AM trade study: An AM with a single, central, recessed engine, and an AM with six, smaller, side-mounted engines. Each lander design is presented with design assumptions, a configuration description, drawings, and a summary of design advantages and disadvantages. The Boeing lunar lander trade studies have produced a wealth of useful design data and four viable, alternative lunar lander vehicle concepts.